Learn Hardcore German: Ich warte, bis die blinkende Ampel grün wird. - I wait until the blinking traffic light turns green.

In German, any subordinate clause introduced by a conjunction like bis must be separated from the main clause by a comma. So you write:
Ich warte, bis die Ampel grün wird.

Why does the verb wird appear at the end of the clause?

German subordinate clauses always send the finite verb to the end. Since everything after bis forms a subordinate clause, wird goes to the final position:
…bis die Ampel grün wird.

Why use bis instead of wenn here?

Bis means “until” in the sense of “up to the moment when something happens” (the light turning green). Wenn is used for “when” referring to repeated or general events, not for a one-time point you’re waiting for.

What is the role of blinkende in die blinkende Ampel?

Here blinkende is the present participle of blinken, used attributively like an adjective (“blinking traffic light”). It agrees with Ampel (feminine, singular, nominative), so it takes the -e ending.

Why doesn’t grün have an ending in grün wird?

After verbs like werden (to become), adjectives are used predicatively and remain uninflected. Only attributive adjectives (directly before a noun) take endings.

What case is die Ampel in the subordinate clause, and why?

It’s nominative, because die Ampel is the subject of the clause die Ampel wird grün. Feminine nominative singular uses die.

Why is ich warte in the simple present and not expressed like the English continuous form (“I’m waiting”)?

German does not have a separate progressive tense. The simple present (ich warte) covers both “I wait” and “I’m waiting,” with context supplying the nuance.

Could you say … bis die Ampel grün ist instead of wird?

Yes, bis die Ampel grün ist is grammatically correct. However, wird highlights the change of state (turning green), whereas ist simply states the resulting condition.