Thank you, Guyuomar and Irena for those thoughtful comments. I’m going to address SHL first, because he starts his post with the rather nuanced “I disagree.”

SHL:

“The grammer (sic) is just the mechanism by which vocabulary is put together to speak intelligibly.”

Take out the word “just” and we agree. You need grammar to speak intelligibly (and to listen with understanding). Speaking unintelligibly is useless.

SHL continues:

“You can’t compare 500 or 1000 rules of grammar to 60,000 vocabulary words.”

Most native speakers don’t really know sixty thousand  vocabulary words. Many foreign languages learners won’t get to sixty thousand words in their target language — so what? On average, people know on the order of a thousand words for every year they’ve been alive or speaking the language. Certainly children don’t know sixty thousand vocabulary words. But children definitely speak their language as native speakers. Many foreign language learners do acquire a larger vocabulary than that of (an average) native speaker, but again, so what? That is exactly my point — grammar (and pronunciation) is the language — not vocabulary. 

If I may continue SHL’s rather silly house metaphor — vocabulary is just a pile of bricks; you need to put them together to make a house. The style of house is the specific language or idiom.

SHL: Let’s put it this way:

Every speaker’s vocabulary is unique in terms of the number of words. Some natives know 5000 words, some non-natives know 100000. (It depends not just on your definition of “word,” but also on your definition of “know.” People generally have one or two words in passive vocabulary for every word in their active vocabulary. Over forty percent of the words used by Shakespeare only show up once in all his works. That’s right — he only used them once.)

On the other hand, every five-year-old native speaker’s usage of the real grammar (descriptive grammar) is essentially the same. That is the language. That basic grammar is used in every single sentence, unlike most vocabulary words after the first seven or eight thousand, which get used once in a blue moon (i.e. “not often”). Foreigners who aren’t able to use that real grammar easily in conversation don’t know the language, regardless of how many words they may have memorized.

Languages borrow (and don’t return) vocabulary all the time, but they rarely borrow grammar. Spanish has thousands of words from Arabic, for example.

One more thing — Chomsky is just a mortal — not everything he says is true. In fact, his theories, while extremely interesting, are merely theories.

@Guyomar 

You know, despite the fact that I've read thousands (tens of thousands?) of pages of French literature, I still can't conjugate regular verbs in the passé simple. I know what to do with regular verbs that end in -er for the third person singular (parler -> il/elle parla), but that's the end of it. (Interestingly enough, this causes no problems with comprehension. I know the passé simple when I see it; I just can't use it.) If I ever decide that I'd actually like to be able to use the passé simple in my own writing, I'll have to sit down with a grammar book and memorize the conjugation endings. 

I agree that memorizing grammar rules is not enough, but at least for me, it seems to be a prerequisite for actually acquiring those rules. 

@Phil 

@SHL 

If we're to think of a language (or rather: a non-native speaker's command of the target language) as a house, then any house will contain basic grammar (roughly, a decent approximation of what 5-year-old native speakers implicitly know) plus basic vocabulary. If that (basic grammar and basic vocabulary) is all you know, then you've got a wooden hut, with no running water and no electricity. To turn that hut into a palace, you need a bit more grammar (e.g. literary tenses) and massive amounts of vocabulary. 

Basic vocabulary alone is a cardboard box (for sleeping under a bridge, or more to the point, for asking and answering very basic questions). That's far less functional than a wooden hut, but it sure is better than nothing. If all you have is grammar, that's a house frame. You could, I suppose, take a picture of it and put it in an architecture (or more to the point: linguistics) textbook, but that's about all that it's good for. A lot of vocabulary and no grammar? Ugh. That's a highly unstable house, one that could collapse on you at any moment. (I'm serious. If you know a lot of words, but have poor grammar, you'll keep trying to express complex thoughts, and you'll keep getting puzzled looks from native speakers who cannot figure out what on earth you're talking about.) I'd rather have a wooden hut.