In the sentence What keeps us healthy and happy, what does "s" in "keeps" stand for? it's not a singular verb! I don't understand the usage of "s." Can you explain that? Thanks
Well, yes, it is a singular verb, but the understanding of it as such is idiomatic. It may be helpful to think of this sentence as What thing keeps us healthy and happy? We understand what to be asking about something by default, not some group of things.
In purely logical terms, what could be understood to be asking about a plurality, but again, we don't think of it that way. If we wanted to ask about a plurality, we would specifically ask What things keep us healthy and happy? or What are the things that keep us healthy and happy?
It's a matter of simple grammar, the 's' is added to create the right conjugation. A few examples:
Present tense - I/you/they keep - He/she/it keeps
Past tense - I (have) kept
'What' can be considered singular here, despite the fact that the answer could be Fruits and vegetables keep us happy and healthy.