[rgossiaux]

I think it's best to use native speakers as oracles: you can give them a piece of language and they can tell you if it's correct/natural or how they'd phrase it. You can't expect people to go beyond that, and trying to explicitly systematize the grammar isn't necessarily important or that valuable anyway; you need to internalize the grammar (as one has done with a native language) and that's not a conscious process.

[shard]

I think this really depends on the individual learning style. Some people are able to extrapolate from examples and internalize it (e.g. those people who learn language simply from watching TV), and others need a more explicit structure to guide them before they really start internalizing. No method works for everyone all the time.