First off, sorry if I used non diatonic wrongly in the title. I know that different modes are assigned to different degrees of a scale, like in a major scale I is Ionian, IV is Lydian, etc. Surprisingly, I only recently worked out that this is related to the key, like in the key of G, F isn't Locrian, F# is. But I was wondering then what would I use if I wanted to play a chord that didn't fit into the key. If I was playing in G, and suddenly had an overwhelming urge to play a bar of F using modes, how would I go about that?