Learn Turkish: Kırmızı arabadan önce dur.

The postposition önce (before) always requires the noun before it to be in the ablative case (-dan / -den / -tan / -ten). To say "before the car", you must say arabadan önce.

I learned that önce means "before" in time. Can it also mean "before" in physical space?

Yes! Although you usually see önce and sonra used for time, they can also describe physical sequence or order on a route. In this sentence, it means stopping physically "before" reaching the red car.

Why doesn't the adjective kırmızı get any endings?

In Turkish, case endings like the ablative (-dan) only attach to the noun, never to the adjectives describing it. Adjectives stay in their base form, so it is simply kırmızı arabadan.

What form of the verb is dur?

It is the imperative (command) form for a single person you are informal with. To make this command, you just take the infinitive durmak (to stop) and remove the -mak ending, leaving the bare root dur.