When to use “fahren” and when to use “abfahren”?

Question

I have seen many times these two verbs, for example:

Der Zug fährt ab

Ich fahre nach Deutschland

What are the differences between them and when to use each one?

Thanks

Answer 1

Fahren means to drive. In your example, you're going to Germany. This is the actual movement. Note that this is sometimes generalized to a voyage, so it is also used if you're going by plane.

Abfahren means to depart. In your example, the train departs. This is only the start of the movement and is never used for planes.

Answer 2

fahren is the general verb for drive / go

Ich fahre mit dem Bus. - I go by Bus.
Ich fahre Bus. - I drive a Bus.

If the action of driving is not the main point, you rather use mit, when telling about what vehicle you'll use

Ich fahre mit dem Auto nach Deutschland.

That doesn't tell something about whether you drive it or not. It might be concluded from the context though.

ab has the sense of from / off [ / since]

So abfahren is about leaving from somewhere

Der Zug fährt ab. - The train departs.
Wann fahrt ihr ab[/los]?

There are not many use cases.

Using abfahren with the preposition auf, it has the meaning of a strong affection.

Er fährt auf sowas ziemlich ab

Using it in a predicative way, it has the meaning of exceptional / extraordinary

Das ist ja abgefahren!