Learn German: Sie ziehen sich nach der Arbeit um. - They change clothes after work.

The verb here is sich umziehen (to change clothes), which is a reflexive verb. In German, you have to express that they are changing themselves. The word sich is the reflexive pronoun for "they," acting as the direct object (Accusative case) in this sentence.

I see ziehen and um. Are these part of the same verb?

Yes! The infinitive of the verb is sich umziehen. It is a separable verb, which means the prefix um splits off and moves to the very end of the sentence. On its own, ziehen usually means "to pull," but adding um changes the meaning entirely.

The word for work is die Arbeit. Why does it say der Arbeit here?

The preposition nach (after) always requires the dative case. Because Arbeit is a feminine noun, its article changes from die to der in the dative case.

Since Sie can mean "she," "they," or formal "you," how do we know it means "they" here?

We can tell by looking at the verb ending. If Sie meant "she," the verb would be conjugated with a -t ending: Sie zieht sich um. Because the verb has the -en ending (Sie ziehen), it must be plural ("they") or the formal "you."