Die Ampel blinkt gelb, darum fahren wir langsam.

Because Ampel is a feminine noun in German. Its dictionary form is die Ampel.

In this sentence, die Ampel is also the subject, so it is in the nominative case:

• die Ampel = the traffic light

German main clauses normally follow the verb-second rule.

So in:

• Die Ampel blinkt gelb.

the first position is Die Ampel, and the finite verb blinkt comes second.

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

Because German often uses verbs like blinken and leuchten directly with a color word.

So:

• Die Ampel blinkt gelb = the traffic light is flashing yellow

This is natural German. Compare:

• Die Ampel ist gelb = the traffic light is yellow
• Die Ampel blinkt gelb = the traffic light is flashing yellow

The second one focuses on the flashing signal, not just the color as a static property.

Here gelb does not stand before a noun, so it does not take an adjective ending.

Compare:

• das gelbe Licht = the yellow light
• Die Ampel blinkt gelb = the traffic light flashes yellow

In the sentence, gelb works like a predicative/adverbial color word, so it stays in its basic form.

Here darum means for that reason, therefore, or that’s why.

It introduces the result of the first clause:

• Die Ampel blinkt gelb, darum fahren wir langsam.
• The light is flashing yellow, therefore we drive slowly.

So darum is giving a cause-and-result connection.

Because darum takes the first position in the main clause, and German still keeps the verb in second position.

So the pattern is:

• Darum
  • fahren
    • wir
      • langsam

This is standard German main-clause order:

• first position: darum
• second position: fahren
• then the subject: wir

This is very common:

• Deshalb fahren wir langsam.
• Heute fahren wir langsam.
• Jetzt fahren wir langsam.

Whenever something other than the subject comes first, the verb still stays second.

Not quite.

Darum means therefore / that’s why, and it introduces a main clause:

• Die Ampel blinkt gelb, darum fahren wir langsam.

Weil means because, and it introduces a subordinate clause, where the verb goes to the end:

• Wir fahren langsam, weil die Ampel gelb blinkt.

So the meaning is similar, but the grammar is different:

• darum → main clause word order
• weil → subordinate clause word order

Because the sentence contains two main clauses:

1. Die Ampel blinkt gelb
2. darum fahren wir langsam

The comma separates those two clauses.

It is especially important here because darum is not a subordinating conjunction like weil. It is an adverb inside the second main clause, so the second part remains a full main clause.

Because langsam is describing the verb fahren, not a noun.

So here it functions like an adverb:

• wir fahren langsam = we drive slowly

Compare:

• ein langsames Auto = a slow car
• Das Auto fährt langsam = the car is driving slowly

Before a noun, you need an adjective ending. When describing the action, you do not.

Yes. All of these are common and natural:

• Die Ampel blinkt gelb, darum fahren wir langsam.
• Die Ampel blinkt gelb, deshalb fahren wir langsam.
• Die Ampel blinkt gelb, deswegen fahren wir langsam.

They are all very close in meaning. For a learner, it is usually safe to treat them as near equivalents.

The word order stays the same:

• Deshalb fahren wir langsam.
• Deswegen fahren wir langsam.

German uses the present tense very often for actions happening now.

So:

• blinkt can mean blinks or is flashing
• fahren can mean drive or are driving

German does not need a separate form like English is flashing or are driving. The normal present tense often covers both ideas, and context tells you which one is meant.

Yes, very normal.

For traffic signals, Germans commonly talk about the light this way:

• Die Ampel ist rot.
• Die Ampel wird grün.
• Die Ampel blinkt gelb.

So this sentence sounds natural and idiomatic.