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’. In the grammar of putting words and parts of words together to form phrases and sentences, the syntax thus takes account of both the order of the words and their forms. The term syntax is from ancient Greece. Syntax determines whether the way words are put together makes sense, or not. To a large degree, the sense involves what are known as the ‘subject’ and ‘object’ with respect to the verb. English is particularly simple and 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” where the dog is female.

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.

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.