How much does knowledge of linguistics help with learning foreign languages?

[FlatAssembler]

Think back to childhood, when you learned your first language: Unless you lived in a very odd household, chances are someone didn't lecture you on phonemes and declensions and such -- they just spoke to you, and eventually you made a transition from speaking to writing.

I agree with you on that. Learning syntax and morphology in details doesn't really help you speak a language grammatically, and it could actually be counter-productive. But, if you already know a closely related language, knowing the sound changes might help you remember the vocabulary.