Never, ever, ever pretend to know. The person you are talking to will instantly know that you are pretending and in that moment your credibility is gone. Getting it back is next to impossible.
It is very okay to admit you don’t know and you will find out and get back to your prospect or client with the full answer. Your credibility account will actually get a nice deposit!
When you are seeking the information you need, to be able to answer the question the has been asked of you, this is best way I know for you to never have to pretend again. Just keep asking for clarity until you reach the point where you are 100% certain you could explain what you have learned to someone else.
Depending on the complexity of what you have found out, it is also a good idea to practice your explanation with a peer. Then you will truly know what you know.