Why does a lightning flash come before the thunder?

Because both start at the same instant and light gets to you first. Light covers a few kilometres in a fraction of a millisecond; sound takes about three seconds per kilometre. Counting the seconds and dividing by three gives the distance in kilometres, and by five for miles.

The thunder is the air itself. A lightning channel is heated to around 30,000 degrees - roughly five times the surface of the Sun - in a few microseconds, and the air explodes outward as a shock wave that decays into sound. A nearby strike gives a single crack because the whole channel is about the same distance away; a distant one rumbles because sound from the near and far ends of a channel several kilometres long arrives spread over seconds.

Beyond about 15 kilometres you usually see the flash and hear nothing, because the sound is refracted upward by warmer air near the ground - the old name for that is heat lightning, and it is not a separate phenomenon. If you can hear thunder at all you are within range of being struck, which is the entire basis of the standard safety advice.