But they also do a great deal more than that. One of the areas where auxiliary verbs reign supreme is in giving people street cred. Which of the following sentences was said by a teenager to the gang?
I'm thinking we'd better do it this evening.
I am thinking we had better do it this evening.
And who do you think might have said the other? Ask a teenager and she'd say a teacher or academic. Most auxiliary verbs have two forms, one short and the other even shorter: they are they're you have you've she will she'll
We generally write the short one with an "apostrophe of omission". We make the same choice when we add not to an auxiliary verb: they are not they're not or: they aren't you have not you've not or: you haven't
Reduced forms are normal in casual speech, so these are the forms that children are most familiar with. But writing is different. In writing we always use the full form - except when we don't. This is one of the hardest choices to learn and to teach because it comes down to style rather than hard-and-fast rules. What should you tell the children?
Our preferred option is to see reduced forms as an excuse for exploring genres. What do successful writers do in different text-types? Take a written text - a textbook, a novel, a poem, a sports commentary, even a TES grammar column - and check for reduced forms. You already know the outcome: most texts reduce some auxiliaries but not all of them, and what distinguishes texts is not whether, but how often, they reduce.
How? Count the verbs that could have been reduced, and see how many actually are. (For example, in this column, excluding examples, you'll find we reduced 616 = 37.5 per cent.) Just think of the skills that would grow in this kind of text-comparison: literacy, numeracy, science and problem-solving all rolled into one.
In the process, students will learn that auxiliaries have an important stylistic purpose, helping us to communicate our tone, and suggesting our attitude towards topic and audience. They'll also see that the stereotypes of who uses reduced forms and when aren't quite as predictable as we might assume. That, in itself, is a useful lesson in mistrusting stereotypes.
Richard Hudson is professor of linguistics at University College, London. Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk