For example, here are two important facts about nouns and adjectives:
* A noun (unlike a verb) can be modified by an adjective; so we can modify the noun "arrival" by adding the adjective "recent", giving "recent arrival", but we can't do the same with the verb "arrive" - we can't say "He recent arrived."
* An adjective (unlike an adverb) can be used to modify a noun; so as we've just seen, you can use "recent" to modify "arrival", but you can't use "recently" to give "recently arrival".
When you're teaching about nouns you need to talk about adjectives, but you can't teach about adjectives without talking about nouns - and at the same time you really need to contrast these word classes with verbs and adverbs.
Everything depends on everything else. In short the word classes are not a mere list, but a complex network.
The network has links of different kinds, which we can distinguish and name - just like a social network where we distinguish friends, colleagues, neighbours and relatives. Other links are called "subject" and "object", but grammar doesn't exist in a vacuum so words aren't just linked to other words; they're also linked to meanings, forms (word-parts such as affixes), speakers, languages (eg all the words in this column are English) and so on.
This is where a "naming of parts" approach to grammar runs into problems, and why the best grammar teaching has to engage at some point with real texts. But just like the real world, texts can prove messy and difficult to dissect neatly. Teaching one small part of grammar without reference to another can cause confusion.
That's why grammar is so interesting - one thing leads to another. You can't look at one word without noticing its similarities and connections to other words, and you then realise that all these links are in your mind.
But that's why it's so hard to teach - how do you turn a network of ideas into a lesson plan? It can be done, of course, but it's a challenge to the teacher's ingenuity.
One thing is clear - it won't do to treat grammar just as "naming of parts", consisting of a list of boxes with simple "notional" definitions like "naming word". To teach it like this is to miss the whole point.
But the benefit of teaching grammar as a network - apart from increased understanding of our own language - is a process of deep learning rather than shallow memorisation. In other words, grammar is helping our students to improve their thinking skills. The power of networking shouldn't be underestimated.
Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk