Teaching about writing is all about choices and their effects. In a nutshell, writing has two parts: choosing the words, and then choosing the letters.
Choosing the words is the tricky bit because different choices have different effects, and there's no super-dictionary where we can look up the effect we want and find a single "correct" word.
Words and effects are related in complicated ways.
You can see this as a problem, or as a blessing. It's a problem if you want to train children like little computers who know exactly how to react to every demand. But then, you probably don't think of children like that, so it won't worry you. So let's think of it as a blessing: this is what it means to say that language is a wonderful resource for expressing complex ideas.
Here's an example of the complicated relations between words and their effects. It involves the order of words rather than the choice of individual words, but ultimately it's still a matter of choosing words: which word shall I write next?
If you take back a present, you're also taking the present back. The crucial words here are "take back", which make up a phrasal verb. (We talked about these in March 2003; you can find it at www.phon.ucl.ac.uk homedickTES.htm). A phrasal verb comprises a verb and a "particle" (which is often a preposition).
What's crucial about phrasal verbs is that the particle may stand either before or after the verb's object: so it's either "take back a present" or "take a present back".
But just look at how many effects this one choice can have.
* Effect: ungrammatical. If the object is a pronoun (eg "it"), the choice makes a great deal of difference. Try it: "I took it back." "I took back it." There's no question: the second simply isn't English.
* Effect: unreadable. For maximum readability, keep the verb and particle near to each other. Here's the effect of ignoring this advice: "I took the present I had just given her to celebrate her promotion back."
* Effect: ambiguous. Particles can also modify nouns, so your ticket back is your ticket for getting back. Now spot the ambiguity in "I took my ticket back."
* Effect: unambiguous. Most particles can also be used as prepositions, so if you look up a road you could be looking up it (where are you looking?) or looking it up (finding it in an index). To be absolutely clear, you write: "I looked the road up."
So: one grammatical pattern, many different effects. This is why grammar can never be reduced to lists of patterns and their effects. It's much more subtle than that, and more enjoyable.
Richard Hudson is professor of linguistics at University College London
Geoff Barton is headteacher of King Edwatd VI School, Bury St Edmunds, Suffolk