Welcome to the forum.
Don't bluff your way through it! You'll get caught out. I suggest something like "That's a great question. It's not the topic of today's lesson but how about I put together a lesson on it for next week/next month?"
Remember that you are the person who dictates the topic of the class. It's easy to get dragged off track by questions - good, bad or average.
You're absolutely right that your students think there is nothing you don't know. That's just the way it is. However, you know it's not possible for anyone to know all the rules, or to instantly be able to explain them and come up with examples.
There is very little point explaining a grammar rule unless you have time to go into it properly, with examples and exercises. You can explain to that student that it would be of no benefit to anyone to half-explain something.