Learn German: Bitte fahr das Schild nicht um, falls du hier parkst! - Please do not run over the sign if you park here!

The clue is that the prefix is separated (fahr ... um). When umfahren means "to run over" or "knock down", the prefix um- is separable. If it meant "to drive around" or "bypass", the prefix would be inseparable, and the command would look like Bitte umfahre das Schild nicht!

Why is the command form fahr instead of fähr?

This is the informal command for a single person (du). To form it, you normally take the present tense du form (fährst) and drop the -st ending. However, strong verbs that take an umlaut in the present tense always drop that umlaut in the command form, so fähr goes back to fahr.

Why is nicht placed right before um?

In German, nicht likes to go as close to the end of the sentence as possible to negate the whole action. However, a separated prefix like um always claims the absolute final position in a main clause. Therefore, nicht takes the next best spot right in front of it.

Could I use wenn instead of falls?