So when negating być, you still use instrumental, not genitive? Does this hold for all instrumental verbs?
You only use genitive in a negative sentence when the positive sentence would use accusative.
Thanks, that makes sense. Also I suppose when the positive sentence uses the genitive?
Yes, whoops! Scratch that "only" :)
Edit: To sum up, if the positive sentence uses accusative, you use genitive in the negative sentence. For other cases they stay the same.
Really. We don't usually get into the colloquial territory.
The Polish equivalent of "cop" is "glina" or "gliniarz".