Sometimes, especially on this forum, I am wondering wether I should say "what does it mean" or "what does it means" ? And should I say "it means that..." or "it mean that..." ?
Here are the relevant rules:
You need subject-verb agreement.
What does it mean?
It is the logical subject (it does mean what), and does is the conjugated verb, so it does (mean), does with an S.
It means that...
It is the subject, so means with an S.