Grammar ''finite verb'' ...

Hi,

I would like to know if the following is correct?

I made him feel happy yesterday.

Is it feel instead of felt?

This is the reason (why) I left you.

Do you need the word why?

The reason I left you is/was that you were.....

Both is and was can be used depending on the context. Am I right?

Also, is it I have no idea or I have no ideas ?

And, let's say you are riding a bike and you're under the legal age. You knew it was wrong beforehand. So, while riding the bike, you say

a) I knew this was a bad idea.

b) I knew this is a bad idea.

c) I know this is a bad idea.

I would pick b) as the action is still taking place.