Question

If you are working in C Major, and use the IV (F) in a bar, do you use the C-Major or F-Major key signature for this?

Answer 1

I'm not sure what you mean. The key is still C major, but you are a playing F major. The notes of F major are all in the C major scale.

Only if you "modulate" at that point would you change key signatures.

Answer 2

Believe it or not, you've helped me. Thanks.