Learning the tools
A plumber who knows all his tools by heart, but no more, isn't a plumber. He also has to learn how to use the tools - their functions. What kinds of thing can you do with a spanner? And the other way round: if you want to turn a nut, what tools can you use? In short, he learns the functions of his tools.
What he has to learn is complicated because he can use his spanner for hitting things as well as for turning nuts, and he can use a wrench as well as spanner for turning nuts - forms and functions don't match neatly.
It's exactly the same in writing. The main tools of the trade are words and more general grammatical patterns - what we can call "forms" - and we use forms to achieve effects, so they have "functions". And just as in plumbing, one form can have many functions and vice versa. Fortunately, most of our apprentice writers already have a rich kit of multi-functional tools, but our discourse about this kit doesn't always do it justice.
Take the term "question", for example. Is this a form or a function? Is a question a grammatical pattern, a kind of sentence - ie, a form - or is it a kind of meaning - a function? To see that this choice matters, try to decide which of the following sentences is a question. (We've deliberately chickened out of punctuating the examples so you're not influenced by our use of question marks.)
1. How do you open it
2. How dare you
3. Would you pass the salt please
4. Why don't you phone him
5. What, it's your birthday today
6. I wonder how you open it
7. Tell me how you do it
We can all agree that example 1 is a really good question, but from then on it's tricky. If we define questions in terms of grammatical form, then a sentence is a question if its subject follows the auxiliary verb (eg do you, dare you, would you); by this criterion, 2-4 are questions but 5-7 aren't. On the other hand, if a question is a way of asking a question, ie asking for information, then the questions are 5-7 but not 2-4.
This is why grammarians (and the Literacy Strategy glossary) have two separate sets of terms for classifying sentences, one for their forms (declarative, interrogative, imperative) and the other for their functions (statement, question, request, command, invitation etc). This parallel terminology should be tucked into every writer's toolkit.