How do I know when to use 'for' instead of 'of', and vice-versa?

Question

For example: Prime minister FOR Australia vs. Prime minister OF Australia.

Pretty confusing, aha. ;p

Answer 1

FOR is used more for a reason or a cause while OF is used to describe belonging and physical orientation

for example: Prime minister OF Australia (because he belongs to Australia) while :

I went to library for reading (implies reason)