Use "I'm not quite sure..." to politely say that you don't know something. For example:

A: Where's Doug?

B: He left a few minutes ago. I'm not quite sure when he's coming back.

I'm not quite sure what I'm going to order.

"I'm not quite sure" is followed by a question word like "who", "what", "when", "where", "why", or "how".

It's also possible to follow it with "that" and then a statement, like this:

I'm not quite sure that I'm ready to start dating again.

This kind of sentence sounds a lot more formal than the ones with question words, though.