Learning French in English

I am a good writer (sometimes) and do well in English, but when a teacher asks me to name something that should be included in descriptive writing, or tells me to use a simile, or scribbles 'Good complex sentence!' in the margin of my exercise book, I am simply at a loss for what it all means. I learned all these things once, when I hated grammar, and now I know what they are so well it is almost as if I have forgotten them - it is certain that I rarely think of them when writing.

It's like learning French. Instead of learning it like all the French kids must have learnt it - slowly, from birth, with others speaking it all around them - we learn it through lists of words and what-it-means-in-English and past participles and present, perfect, imperfect, future, conditional and imperative tenses, not to mention the present subjunctive. All of these, our teachers assure us, we have encountered in our own language of English. 

But have we?

Well of course we have, but not knowingly. These tenses are things we only begin to learn about when we start learning another language; we have never been taught about them in our own. And now the question arises: if we don't need to know what they are in English, do we really need to know in French?

And now the question arises: do we all really sit sifting through books for metaphors and alliteration, or do we just... read?