Bevor du nach links fährst, musst du den Blinker setzen.

Because bevor is a subordinating conjunction. In German, a subordinate clause normally sends the conjugated verb to the end.

So:

• du fährst nach links = normal main clause word order
• bevor du nach links fährst = subordinate clause, so fährst moves to the end

This is very common with words like weil, dass, wenn, obwohl, and bevor.

Because the sentence starts with the subordinate clause:

• Bevor du nach links fährst, ...

When a German main clause comes after something else in first position, the verb must still stay in the second position. That means the finite verb comes before the subject here:

• Bevor du nach links fährst, musst du den Blinker setzen.

You can think of the whole bevor-clause as taking position 1, so musst has to come next.

Compare:

• Du musst den Blinker setzen.
• Bevor du nach links fährst, musst du den Blinker setzen.

Yes, bevor means before when introducing an action that happens earlier than another action.

In this sentence:

• Bevor du nach links fährst = Before you drive/go left
• musst du den Blinker setzen = you must use/turn on the indicator

So the sentence expresses the order of actions: first signal, then move left.

A key difference from English is grammatical: after bevor, German uses subordinate clause word order, with the verb at the end.

Nach links means to the left and emphasizes movement in that direction.

• nach links fahren = to drive to the left / move left
• links by itself can also be used adverbially, especially in phrases like links abbiegen = turn left

So both ideas exist in German, but they are used in slightly different patterns:

• nach links fahren
• nach links wechseln
• links abbiegen

In this sentence, nach links fits the verb fahren naturally.

Literally, den Blinker setzen means something like set the indicator. In natural English, that usually means:

• use the turn signal
• put your indicator on
• signal

Yes, it is a real German expression, especially in driving contexts, though some learners may hear these alternatives more often in everyday speech:

• den Blinker anmachen = turn on the indicator
• blinken = signal / indicate
• den linken Blinker setzen = put on the left indicator

So den Blinker setzen is correct and understandable, but in casual speech many people might also say den Blinker anmachen.

Because Blinker is the direct object of setzen, so it takes the accusative case.

• nominative: der Blinker
• accusative: den Blinker

The verb setzen here is acting on something: you set/use the indicator. That makes Blinker the object.

This is why the sentence says:

• du musst den Blinker setzen

not

• du musst der Blinker setzen

Because German normally separates a subordinate clause from the main clause with a comma.

Here the subordinate clause comes first:

• Bevor du nach links fährst,
  musst du den Blinker setzen.

This comma is required in standard German.

Yes, very often you will hear:

• Bevor du nach links abbiegst, musst du den Blinker setzen.

That means Before you turn left, you must signal.

The difference is:

• nach links fahren = drive/go to the left
• nach links abbiegen = turn left

In many traffic situations, abbiegen is the more precise verb if you mean making a left turn at a junction. But nach links fahren can still work depending on context.

Yes, it could also be Sie.

• Bevor du nach links fährst, musst du den Blinker setzen.
• Bevor Sie nach links fahren, müssen Sie den Blinker setzen.

Du is informal singular.
Sie is formal singular or plural.

Which one is used depends on context. In learner materials, instructions are often given with du because it is simpler and more direct.

Yes. müssen means to have to / must.

So:

• du musst = you must / you have to

In this sentence, it expresses a rule or obligation in traffic.

German müssen is very common for rules, necessity, and strong obligation.

It can be rearranged. You can also put the main clause first:

• Du musst den Blinker setzen, bevor du nach links fährst.

This means the same thing.

The main difference is word order in the main clause:

• If the sentence starts with du, you get du musst
• If the sentence starts with the bevor-clause, you get musst du

So both are correct:

• Bevor du nach links fährst, musst du den Blinker setzen.
• Du musst den Blinker setzen, bevor du nach links fährst.