People often think of grammar as a matter of arbitrary pronouncements (defining 'good' and 'bad' language), usually negative ones like There is no such word as ain't or Never end a sentence with a preposition. Linguists are not very interested in this sort of bossiness (sometimes called prescriptivism). For linguists, grammar is simply the collection of principles defining how to put together a sentence.

 One sometimes hears people say that such-and-such a language 'has no grammar', but that is not true of any language. Every language has restrictions on how words must be arranged to construct a sentence. Such restrictions are principles of syntax. Every language has about as much syntax as any other language. For example, all languages have principles for constructing sentences that ask questions needing a yes or no answer, e.g. Can you hear me?, questions inviting some other kind of answer,e.g. What did you see?, sentences that express commands, e.g. Eat your potatoes!, and sentences that make assertions, e.g. Whales eat plankton.

Word Order

 The syntactic principles of a language may insist on some order of words or may allow several options. For instance, English sentences normally must have words in the order Subject-Verb-Object: in Whales eat plankton, 'whales' is the subject, 'eat' is the verb, and 'plankton' is the object. Japanese sentences allow the words to occur in several possible orders, but the normal arrangement (when no special emphasis is intended) is Subject-Object-Verb. Irish sentences standardly have words in the order Verb-Subject-Object.

 Even when a language allows several orders of phrases in the sentence, the choice among them is systematically regulated. For example, there might be a requirement that the first phrase refer to the thing you're talking about, or that whatever the first phrase is, the second must be the main clause verb.

Not only does every language have syntax, but similar syntactic principles are found over and over again in languages. Word order is strikingly similar in English, Swahili, and Thai (which are utterly unrelated); sentences in Irish are remarkably parallel to those in Maori, Maasai, and ancient Egyptian (also unrelated); and so on.

Word Structure

 However, there is another aspect of grammar in which languages differ more radically, namely in morphology, the principles governing the structure of words. Languages do not all employ morphology to a similar extent. In fact they differ dramatically in the extent to which they allow words to be built out of other words or smaller elements. The English word undeniability' is a complex noun formed from the adjective 'undeniable', which is formed from the adjective 'deniable', which is formed from the verb 'deny'. Some languages (like German, Nootka, and Eskimo) permit much more complex word-building than English; others (like Chinese, Ewe, and Vietnamese) permit considerably less.

 Languages also differ greatly in the extent to which words vary their shape according to their function in the sentence. In English you have to choose different pronouns ('they' versus 'them') for Subject and Object (though there is no choice to be made with nouns, as in Whales eat plankton). In Latin, the shapes of both pronouns and nouns vary when they are used as subjects or objects; but in Chinese, no words vary in shape like this.

 Although we have identified some differences between syntax and morphology, to some extent it is a matter for ongoing research to decide what counts as morphology and what counts as syntax. The answer can change as discoveries are made and theories improved. For instance, most people--in fact, most grammarians--probably say that 'wouldn't' is two words: 'would' followed by an informal pronunciation of 'not'. But if we treat 'wouldn't' as one word, then we can explain why it is treated as one word in the yes/no question Wouldn't it hurt? Notice that we don't say Would not it hurt? for Would it not hurt?, or Would have he cared? for Would he have cared? In each case, the bad versions have two words before the subject.) The syntactic principle for English yes/no questions is that the auxiliary verb occurs before the subject.

 If this is correct, by the way, then 'ain't' certainly is a word in English, and we know what kind: It's an auxiliary verb (the evidence: We hear questions like Ain't that right?). English teachers disapprove of 'ain't' (naturally enough, since it is found almost entirely in casual conversation, never in formal written English, which is what English teachers are mostly concerned to teach). But linguists are generally not interested in issuing pronouncements about what should be permitted or what should be called what. Their aim is simply to find out what language (including spoken language) is like.

 Even if you learned all the words of Navajo, and how they are pronounced, you would not be able to speak Navajo until you also learned the principles of Navajo grammar. There must be principles of Navajo grammar that are different from those of other languages (because speakers of other languages cannot understand Navajo), but there may also be principles of universal grammar, the same for all languages. Linguists cannot at present give a full statement of all the principles of grammar for any particular language, or a statement of all the principles of universal grammar. Finding out what they are is a central aim of modern linguistics.