Learn Hardcore German: Sobald die Ampel grün wird, fährt das Taxi los. - As soon as the traffic light turns green, the taxi drives off.

Sobald is a subordinating conjunction meaning “as soon as.”
• It introduces a subordinate clause describing a moment in time.
• The main clause follows after a comma.
• In the subordinate clause, the finite verb goes to the end.

Example structure:
[subordinate clause], [main clause]
Sobald die Ampel grün wird, fährt das Taxi los.

Why does wird come at the end of the Sobald-clause?

Because Sobald is a subordinating conjunction, it sends the finite verb to the final position in its clause.
In subordinate clauses introduced by words like weil, dass, obwohl, and sobald, German uses the Verb-final rule:

[Sobald + subject + other elements + verb]

Here:
Sobald (conjunction) + die Ampel (subject) + grün (predicate) + wird (finite verb).

Why is the article die used with Ampel?

Because Ampel (traffic light) is a feminine noun in German.
All nouns have grammatical gender (masculine, feminine or neuter).
• Feminine nouns take die in the nominative singular.

die Ampel (the traffic light)

What exactly does wird grün mean? Why not just ist grün?

• ist grün → “is green” (describes a state)
• wird grün → “turns green” or “becomes green” (describes a change of state)

German often uses werden + adjective to express the idea of becoming or turning into something.

How does fährt ... los work? Is losfahren a separable verb?

Yes. losfahren is a separable prefix verb meaning “to depart” or “to drive off.”
• In main clauses, the prefix los- detaches and goes to the end.
• The finite verb fährt stays in second position, and los goes to final position:

fährt (finite) … los (separable prefix)

Thus:
fährt das Taxi los → “the taxi is taking off / departing.”

Why is there a comma before fährt?

German requires a comma between a subordinate clause and the main clause.
Since Sobald introduces a subordinate clause, you must separate it with a comma:

Sobald die Ampel grün wird, ⟵ comma
fährt das Taxi los.

Why is das Taxi in the main clause with the definite article das instead of ein Taxi?

Using das Taxi (the taxi) suggests a specific or already-known taxi, perhaps one you are watching.
If you said ein Taxi, it would be indefinite: “as soon as a taxi turns green, it drives off,” which sounds odd since a taxi doesn’t turn green.

Why is the present tense used here to talk about an event that will happen in the future?

German often uses the simple present (Präsens) to express near or certain future events, especially when accompanied by time clauses like sobald, wenn, morgen, etc.
So wird … fährt are both in the present tense but refer to a future sequence:
“As soon as the light turns green, the taxi will pull away.”