Whenever I see 「よく」, I have to ask someone whether it means "often" or "good".
For example, here is a sentence from my book:
I thought all three 「よく」s meant "often" (eg. "often takes notes"), but a Japanese friend said they all mean "good" (eg. "takes good notes"). He said "often takes notes" would be 「よくノートを取り」. However, he doesn't know why.
How can you tell the two usages apart?