5. Grammar matters, but not as much as anal grammar Nazis think it does:
The problem with grammar is that here in the US at least, schools do such a horrible job of teaching the subject that most people are entirely out to sea regarding correct usage. It's the calculus of liberal arts subjects. But grammar need not be stupendously complicated; in the final reduction the point of grammar is to make the language as clear to as many people as possible. Frankly, I think if most non-writers can manage to get agreement between their verb and their subject, I'm willing to spot them the whole "who/whom" conundrum.

Now, obviously, you should know as much grammar as you can; the more grammar you know, the better you can write. But the bottom line is just this: Be as clear as possible. If you're not confident about the grammar of a sentence, re-write it and strive for clarity. Yes, it's possible that in doing so the resulting sentence will lack style or something. But it's better to be plain and understood than to have people admire your style and have not the slightest idea what you're trying to say.