In languages like English, meaning is mainly encoded by the order of the elements, known as ‘constituents’ as well as by the meanings of words and parts of words such as -es in houses, and by the form of other expressions including idioms. This ordering is known as ‘syntax’. Syntax determines whether the way words are put together makes sense, or not. In the assembling of words and phrases and parts of words, the syntax thus takes account of both order and forms.

The term syntax is from ancient Greece. To a large degree, the sense involves what are known as the ‘subject’ and ‘object’ with respect to the verb. English is rather rigid on this point. So with the dog, ate, and the medicine, as three constituents, “The dog ate the medicine” is both plausible and syntactically correct as a sentence. But with ate in any other position, the structure is no longer interpretable. The significance of the subject and object roles is pointed up when these roles are expressed by pronouns, as by “She ate it” or “He ate it”.

The relations between subjects, verbs, and objects are quite complex, sometimes encoded in metaphor. In some cases the meaning of the verb depends on its relation with subjects and objects, mainly the latter. In “I just killed the wasp” and “I just killed a pork pie”, the verb kill means two quite different things. In the second it is a metaphor only in the very extended sense that the pie no longer exists as a pie. But in “The pork pie just killed the patient” the meaning is entirely literal if the patient was almost at the point of death from starvation or if the pie was poisoned or if the patient was critically allergic to some ingredient.

English is characterised as a Subject Verb Object or SVO language. This is not the only possible order. There are slightly more SOV languages than SVO language. VSO languages are unusual. And languages with the subject after the object are rare. But the ordering here is a point which the learner has to attend to. Most languages allow some freedom with respect to word order, as in “The medicine, the dog ate yesterday’, said as a statement of fact, with an emphasis quite different from “The dog ate the medicine yesterday”. The main freedom with respect to word order in English is with to adverbs like yesterday, and adverbial phrases like ‘the day before yesterday’, ‘a few days ago’, and so on. So “Yesterday the dog ate the medicine” is perfectly grammatical. But only in very small number of languages is it possible to break up the constituents, other than to express some very marked meaning, as by “The dog ate the yesterday medicine” if the prescribed medicine varies day by day.

Consider the sentence, “She ate the medicine”. This means something different to “She eats the medicine” and “Doesn’t she eat the medicine?” and “Did she eat the medicine?” and “She didn’t eat the medicine” and “Didn’t she eat the medicine?” These variations are part of syntax, as one key aspect of grammar. Syntax involves words like do, does and did, and the contractions with not as n’t, as don’t, doesn’t and didn’t, and the corresponding forms with will, can, may, shall, and so on. Native speakers of English are expected to know the permitted combinations. Not all the ‘logically possible’ combinations are permitted. “Mayn’t I go?” is just not grammatical, at least not in my variety of English, though it may be somebody else’s, But “May I not go?” is grammatical, at least for me. There is a gap with respect to the negative contraction after may. In potentially hostile situations, in an argument with the border force or with another shopper about who had the first touch on the last item in a sale, a syntactic error could have physical consequences, including arrest, detention, or a broken nose.

In eats and ate there are different and very specific relations between these forms and eat. Eats and ate are thought by most linguists to contain more than one ‘morpheme‘ or minimal element of meaning. In eats there are three elements, in the root form of the word itself, in the marking of singularity, and in the marking of present tense. In ate there is the underlying root form, a change in the vowel, and the marking of the past tense. The rules and principles of syntax are quite different from those of conversation.

In a simple statement involving the three constituents, the dog, ate, and the medicine, the case is clear. There is no difference in logic. All orders are equally logical. In languages such as Russian the ordering is much freer, albeit with subtle changes in the meaning. In English one order is strongly forced by syntax. So the ordering of the main constituents of a sentence is relatively easy for English children to learn. And English children mostly get this ordering right as soon as they start combining the words – usually leaving out the word the.

In English, as in all languages, there are various ways of subtly changing the emphasis, as by “The medicine was eaten by the dog” or “What the dog ate was the medicine” or “It was the medicine that the dog ate”, all of these only learnt long after the simple order.

Syntax thus helps to make meanings exact. But it can also create double meanings, as by Noam Chomsky’s now famous example “Flying planes can be dangerous.”

For the learner of English there is:

• Relatively fixed word order with prepositions before the word they govern as in “on Monday” and with verbs before their ‘complements’ as in “ate the medicine”, but with the opposite order in “a man eating tiger” (or the implausible “a medicine eating dog”);
• The words who, what, which, when, why, how, all pronounced on the left edge of the sentence in simple questions, but understood on the right, as in “Who did you see?” or on the left edge of the clause in  “I know who to see”, but with a quite different meaning in “You saw who?” where there is no question, but only surprise. By the analysis assumed here, what can get DISPLACED from the position where it is understood, as shown by the plausible answers “I saw you” or “I saw nobody”
• A very intricate ‘auxiliary’ system with have, can, do, may, and the past tense forms had, could, did, might, ought (from owed) with varying meanings, involving permission, possibility, time and relevance to the present, and negation, question formation, variations in the agency by the use of the ‘passive’, as in “It might have been being eaten” – by complex manipulations of the structure. The main manipulation obvious to small children learning English is the position of words like can in statements like “You can do that?” and questions like “What can you do?” and “Can you do that?” and negative questions like “Can’t you do that?” On the simplest analysis, the relation between the statements and the questions is expressed by a displacement – in the one case of what and in the other of can.

In the framework here, displacement is pervasive in human language. For language to be learnt the way it demonstrably is, fast and reliability, learning must proceed on the basis that the learning mechanism is highly sensitive to any evidence of displacement. By the analysis of Julie Ann Legate (2002) the scope and role of displacement is very variable across languages. Necessarily it has to fall within the learnability space.

What falls within the learnability space varies inversely with universals. The more of the grammar which is defined by universals, the less the child has to learn. Some languages mark negation by a form at the beginning of the sentence. English marks negation by the difference between “She’s coming” and “She isn’t coming” with not immediately after the form carrying the tense, present or past, in this case present is. Not simple at all.

Displacement and other phenomena of syntax are quite different from the much less systematic but far more extensive processes of discourse and conversation. It is sometimes said that grammar is involved  only in written English, that in spoken English there is just conversation. In my view such assertions are just wrong. Among many other things, conversation involves determining whether the listener might be interested in who ate the medicine or what the dog ate.

In English a small number of forms, S, Z and IZ, denoting plurals and present tense, T, D, and ID denoting past, and ING denoting a continuity, attach inseparably to the right edge of a root. Eight or so systems giving a total of a few hundred irregular verbs like sing, teach, fly, similar in number and complexity to equivalent systems in French, German, Spanish, Italian, Greek, Russian. In English there are just a handful of irregular plurals – in men, women and children. This aspect of the language, including plurals, past tenses, and continuous forms, is discussed here (traditionally) under the heading of morphology.

Much clinical work is done on what is often known as ‘expressive’ or ‘productive’ language, focused on single words and the syntax of subjects, verbs and objects, and some simple aspects of phenomena such as negation, as by “The bird is not in the nest” or “The bird is not in the tree” as opposed to “The bird is in the nest” or “The bird is in the tree”. But this extends only to those aspects of grammar normally being learnt between the first and second birthdays. The typically developing two year old is typically very hard to understand other than to those who know him or her well. And the range of what he or she can say and understand is similarly very limited. Somebody with a grammar extending only to this point would be very limited in what he or she could do, not being able to be trusted with anything as dangerous to him or herself or anyone else as a hammer, a saw or a sharp knife. It would be impossible to explain the potential dangers with sufficient clarity.

English mostly marks shifts of focus or topic by the device known as ‘passivisation’ as by “An asteroid ended the age of the dinosaurs” passivised and questioned in “The age of the dinosaurs was ended? I don’t think so. Modern birds are just highly evolved miniature dinosaurs with forelimbs which are now used as wings or flippers and feathers to keep them warm.” The way that English does passivisation is highly complex, involving the order of the words, the structure of the verb itself with either -en or -ed on the right edge, a special use of the be verb, and the original subject turned into a by phrase. Unsurprisingly this takes a long time to learn, especially when this interacts with questioning and negation as in “Wasn’t the age of the dinosaurs ended by an asteroid?”

Much simpler forms are common in school. But children with language issues, with delayed or disordered language development, often have great difficulties with very common passive forms. In “You were given a book. Where is it?” you were given a book is what is often called a ‘short passive’ – with no by phrase. But if the passive structure is not correctly analysed and understood, the follow up question is incomprehensible.

Consider the sentence, “Give me the toy you had yesterday.” The toy may be just out of the reach of the adult saying it, but within the reach of the child to whom it is addressed. Said the day after a birthday, the reference might plausibly be to a much wanted present the child was given the day before. The child may respond appropriately. But this does not mean that the child has properly understood the sentence. In “Give me the toy you had yesterday” you had yesterday is a what is known as a ‘relative clause’. It restricts the reference down from all the toys in the world or all the toys to which the child has access to one particular toy. So relative clauses are very useful conversational devices. English happens to use them very widely, to both subjects, objects, what are known as ‘indirect objects’ as in “The child you gave a toy to yesterday is coming back to get it”, and it allows the form which introduces the form which introduces relativisation, which or who or where or that to go unpronounced. Some languages allow only the subjects of sentences to be relativised. In such languages “The toy you had yesterday is upstairs” is grammatical”, but “Give me the toy you had yesterday” may be uninterpretable – either because there is no ‘relativiser’ or because the toy is a grammatical object here, or both. Until Syntactic Structures, English relatives were a very complex aspect of grammar. But by Chomsky’s reductionism, it could be seen that the relative clause just ‘recycles’ the structure of the main clause. This is easiest to see in a form less likely to be addressed to a child because it is potentially confusing, “I use the recipe you use” or “I use the recipe which you use”. By some current models of syntax, which is copied from a position as the object of use to a position where it introduces and defines the relative clause as such, and is then optionally left unpronounced. The basic structure of the two clauses is the same. The difference is in the two subjects, I and you. Not simple, but simpler than non-generative alternatives. The principle here is known as ‘cyclicity’. It  minimises repetition.

The clinical utility here is that the reductionism of generative grammar opens the door to a much wider sort of intervention than possible hitherto. Since 1957 this reductionist impulse has been taken forward in a number of ways, by the notion of Merge, reducing  all syntactic operations to the simplest logically possible formation of a binary set of items, from Chomsky (1995), and the theory of Phase, applying syntactic operations in a series of phases, each doing essentially just one thing, from Chomsky (2000). Both of these innovations have been hugely develped in subsequent work. The net effect is to further widen the scope of clinical interventions.