Learn Hardcore Latin: Nemo respondet, quia serva in horto aquam portat. - No one answers, because the female servant is carrying water in the garden.

Because nemo means no one or nobody, and in Latin it is grammatically singular. Since the subject is singular, the verb must also be singular.

nemo = singular subject
respondet = he/she/it responds, 3rd person singular present

So Latin treats no one as one person: nobody rather than a plural group.

Is nemo really just one word meaning no one?

Yes. Latin often uses a single word where English uses two.

nemo = no one / nobody

It is a very common word and already includes the negative idea, so Latin does not need an extra word like non here.

So:

nemo respondet = no one responds
not nemo non respondet

What does quia do in this sentence?

Quia means because. It introduces a clause that gives the reason for what was just said.

So the structure is:

main clause: Nemo respondet
reason clause: quia serva in horto aquam portat

In other words, quia connects the two ideas and shows that the second clause explains the first.