weil. The verb goes last.

weil means "because." It explains why something happens. And it does one thing to the clause that follows: it sends the verb to the very end.

What weil does

weil connects two clauses: the first states what happened, and the weil clause explains why. The moment weil appears, one rule kicks in: the verb leaves its normal position and moves to the very last slot in the clause.

In a normal sentence, the verb sits in position two:

Position 1	Position 2 (verb)	Rest
Ich	lerne	Deutsch.
Er	kommt	nicht.

Add weil and the verb moves to the end:

Main clause	weil	Subject	Middle	Verb (last)
Ich lerneDeutsch,	weil	ich	in Berlin	arbeite.
Er kommt nicht,	weil	er		krank ist.
Sie lernt viel,	weil	diePrüfung		schwer ist.
Wir bleiben zu Hause,	weil	es		regnet.

The comma is not optional

A comma always appears before weil. This is not a style choice. German punctuation requires it whenever a new clause begins with a word like weil. No exceptions.

With modal verbs: infinitive first, modal last

When a modal verb is in the weil clause, the order changes further. In a normal sentence, the modal sits in position two and the infinitive goes to the end. Inside a weil clause, both verbs move to the end, and the infinitive comes first, the modal comes last.

Normal sentence:

Position 1	Modal (pos. 2)	Rest	Infinitive (last)
Ich	muss	heute	arbeiten.
Er	will	das	sehen.

Inside a weil clause, the infinitive arrives first, then the modal verb closes the clause:

Main clause	weil	Subject	Middle	Infinitive	Modal (last)
Ich kann nicht kommen,	weil	ich	heute	arbeiten	muss.
Er bleibt zu Hause,	weil	er	das	sehen	will.
Sie lernt mehr,	weil	sie	diePrüfung	bestehen	muss.
Wir schlafen früh,	weil	wir	morgen früh	arbeiten	müssen.

Also: Modal verbs · dass · denn · nachdem

The pattern in full

Every weil clause follows the same skeleton: subject, everything else in the middle, then the verb or verb cluster at the very end. With a modal verb, the cluster is always infinitive + modal. Without one, the single conjugated verb closes the clause.