Still fuzzy on one point.

After the flop, a new round of betting starts. You can just "check" if you like, unless someone bets something, in which case everyone has to either fold or match/raise that bet. Okay, I'm clear on that. I'm even clear on check-raising and why it pisses the other player off.

But.

Is there a minimum for that after-the-flop bet amount?

For example, if, before the flop, someone had raised from 10 to 20, then does that mean that if you wanna bet after the flop, you gotta bet another 20? Or could you bet a small amount?