If he knew it, he would have told us about it. Is this sentence correct?

Question

Sometimes modal verb and conditional sentence is tricky for non-native speaker like me.

I, you and he met yesterday. He said nothing. I and you (except him) are meeting today again. I don't know if he knew about it or if he didn't know about it, when we met yeseterday. In this situation,

a. If he knew it, he would tell us.
b. If he knew it, he would have told us.
c. If he had known it, he would have told us.

Which is correct? I think a or b, not c.
Sorry for my poor English sentences, but I hope you understand my question. ^^

Answer 1

There is a trick here: "If he knew it" could be a conditional, or it can be past tense. It could mean "if he knew it right now" or "if he knew it yesterday".

Therefore (a) and (b) could be correct: "If he knew it right now, he would tell us", and "If he knew it yesterday, he would have told us". Obviously which one to use depends on the situation.

In (c), "if he had known it", is a conditional and past tense. It can also be used.

Answer 2

According to grammar rules, it is:

If he knows it, he will tell us. If he knew it, he would tell us. If he had known it, he would have told us.

These are strictly followed in English language assessments; in other scenarios, a bit of deviation is OK.