How big is your zone of proximal development? We’re not being personal. We just think all teachers need to know about it.

In case you didn’t cover it at college, one of Vygotsky’s big ideas was that every child has a zone of proximal development, or ZPD. (The TES carried a lucid summary of Vygotsky’s ideas on July 30 - use the advanced search at www.tes.co.uk.) A person’s ZPD is the part of knowledge where they still need a teacher.

In writing, the ZPD is the area of mistakes and anxiety (for example about spelling), but it’s also the point of growth where young writers need sensitive support. For example, most KS3 pupils are still learning to use abstract nouns instead of full clauses, so abstract nouns are in their ZPD.

They’re learning to write “I was surprised by his late arrival” instead of “I was surprised that he arrived late”. The problem is that nouns have their own grammar, with a mass of detail that unforgivingly separates the sheep from the goats. Here are a couple of sentences from a KS3 essay that illustrate the snares lying in wait for the enthusiastic nominaliser: “At first he seems a little excited by the encounter of aliens, ...”; “He also described their difficulty to move or breathe ...” We’ve highlighted two abstract nouns.

Notice how they are right in the middle of the writer’s ZPD. The writer knows the words and uses them. The problem is pure grammar: how to link them to the following phrase. The writer is just guessing, and gets it wrong: “encounter of” and “difficulty to”. We grown-ups know better, of course, but it’s easy to see how the writer hit on those particular words.

After all, “view London” gives “a view of London”, so why shouldn’t “encounter aliens” give “an encounter of aliens”? And if it’s difficult to breathe, why don’t we suffer from difficulty to breathe? An intelligent guess, but wrong.

How can we help in a case like this? It’s not easy - you don’t want to discourage risk-taking in the ZPD, but if it’s in the ZPD that’s precisely because the writer still depends on the teacher for feedback, so we mustn’t just ignore the mistakes.

Worse still, the correct forms are arbitrary - “encounter of” and “difficulty to” might have been correct - so all you can do is show them a pattern. Point out that “encounter” is just like its synonym “meeting” (think of a meeting with the head), and that “difficulty” is like “problem” or “ease” (think of your problems in breathing underwater). As always, the teacher’s main role is to nudge the student to confident and informed experimentation, helping to making connections with other parts of grammar, and giving feedback. The teacher’s role, in other words, is a crucial one.

Richard Hudson is professor of linguistics at University College London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk