Er fährt nicht, weil er betrunken ist.

Because weil introduces a subordinate clause in German. In subordinate clauses, the conjugated verb usually goes to the end.

• Main clause: Er fährt nicht.
• Subordinate clause: weil er betrunken ist

This is one of the most important word-order rules in German.

German uses a comma to separate a main clause from a subordinate clause.

So in:

Er fährt nicht, weil er betrunken ist.

the comma marks the beginning of the weil clause.

This is required in standard written German.

Fährt is the 3rd person singular present tense of fahren.

The verb fahren is a strong verb and changes its stem vowel in the present tense:

• ich fahre
• du fährst
• er/sie/es fährt

So er fährt means he drives / he is driving / he goes by vehicle, depending on context.

Nicht is the general word for not, but its position depends on what is being negated.

In Er fährt nicht, the idea is simply He is not driving / He doesn’t drive in this situation, so nicht comes after the verb phrase core of the short main clause.

German negation is very position-sensitive. Compare:

• Er fährt nicht. = He isn’t driving.
• Er fährt nicht nach Hause. = He isn’t driving home.
• Nicht er fährt. = It’s not him who is driving.

So the placement of nicht changes the emphasis.

Yes, it can be.

Er fährt nicht, weil er betrunken ist. can often be understood as:

• He isn’t driving because he’s drunk.

But structurally, nicht, weil can sometimes also suggest:

• It’s not because he’s drunk that he isn’t driving
  or
• He isn’t driving, but not for that reason

In real life, context and speech stress usually make the meaning clear.

If you want to make the causal meaning extra clear, German speakers often prefer:

Weil er betrunken ist, fährt er nicht.

That more clearly means:

Because he is drunk, he isn’t driving.

Because each clause needs its own subject.

• Main clause: Er fährt nicht
• Subordinate clause: weil er betrunken ist

Even though both clauses refer to the same person, German still states the subject again.

This is normal and works the same way in English:

He isn’t driving because he is drunk.

Because betrunken is being used as a predicate adjective, not an adjective before a noun.

After verbs like sein, werden, and bleiben, adjectives normally do not take endings:

• Er ist betrunken.
• Sie ist müde.
• Das Auto ist schnell.

But before a noun, adjective endings do appear:

• ein betrunkener Mann
• der müde Fahrer

So here, betrunken stays in its basic form.

Yes, but the word order changes.

With denn, the clause after it keeps main-clause word order:

• Er fährt nicht, denn er ist betrunken.

With weil, the verb goes to the end:

• Er fährt nicht, weil er betrunken ist.

So:

• denn = coordinating conjunction
• weil = subordinating conjunction

Both can mean because, but weil is more common in everyday speech.

Yes, absolutely.

That version is very natural and often clearer:

Weil er betrunken ist, fährt er nicht.

When the subordinate clause comes first, the main clause still follows the normal German rule that the finite verb comes second. So after the initial weil clause, you get:

• Weil er betrunken ist, fährt er nicht.

not

• Weil er betrunken ist, er fährt nicht.

That second version would be wrong.

It depends on context.

Fahren can mean things like:

• to drive
• to go by vehicle
• to ride
• to travel

In this sentence, because the reason is being drunk, the most natural interpretation is usually drive:

He isn’t driving because he’s drunk.

But in other contexts, fahren could refer more generally to traveling by car, bus, train, bike, and so on.

It is in the present tense, but like the English present, it can have slightly different uses depending on context:

• right now / in this situation: He isn’t driving.
• general present: He doesn’t drive when he is drunk.

Without more context, German present tense can cover both a current situation and a more general statement.