Here's a problem described by Geoff of Confessions in the comments of an earlier post of mine on grammar:
After I found myself in a French-speaking environment, my latent knowledge was activated and my French took off. But before that, I had a nasty habit of creating sentences that fit the rules but that no native speaker would actually say.And here's Language Fixation on a similar note:
[W]e can all surely think of examples we have heard where someone says something in our native language but it doesn’t seem quite right. Maybe it’s technically correct, but nobody really says it that way.I myself run into this problem all the time in. Take Japanese as an example. I'll say something, completely grammatically correct, only to be informed by my wife that that's not how a native speaker would phrase it. She then tells me how it should be and, little by little, through lots of these short exchanges, I get to sound more and more like a native speaker.
Quite commonly, there are many “grammatically correct” ways to express ideas, but only a few of them are the ones that native speakers actually use. This is really what it means to speak a language… you say what other people say, because you’re used to how it works.
In all of these cases, we're talking about grammatically correct speech. If the speech is truly grammatically correct, why doesn't it sound like native speech? Leaving pronunciation issues aside, that leaves only one obvious culprit: word usage.
What do I mean by word usage? I mean saying it how a native speaker would say it. There are always going to be numerous ways to make yourself understood, but only a fraction of them will actually sound native. Just take a real basic example in English. A native speaker might say, "I'm going to wake up at 7AM tomorrow morning." A non-native speaker might say, "I'm going to arise at 7AM tomorrow morning." Both are grammatically correct. Both convey the same meaning. But the second sounds weird and the first does not. Why? Word usage.
Getting the right word usage down is best done through exposure, both via input and output. Input will let you know how native speakers do it, and output can help as well if someone tells you when you screw up. Just outputting based on what you've learned could, as Geoff describes, result in your own personal pidgin. And that's where the native speaker correcting you becomes extremely useful. You ask them, "Is that how a Japanese person would say it?", and they tell you what you need to do to fix it. A dictionary alone might get you as far as our non-native speaker in the example above, but exposure should be able to eventually take you closer to our native speaker.
Predictably, this is exactly what happened when Geoff got to France.
Picking the right word is, of course, not a grammatical issue at all. So the conclusion that grammar is best learned only through exposure because learning how to pick the right word is best learned only through exposure is a non sequitur.