Learn Spanish (Latin America): Ustedes trabajan temprano en el hospital.

Because the subject is ustedes (you plural). To conjugate regular -ar verbs like trabajar, you drop the -ar and add -an for ustedes, as well as for ellos and ellas (they).

Does ustedes mean 'they' or 'you'?

It means 'you' when talking to more than one person, like 'you all' or 'you guys'. In Latin America, ustedes is the only way to say 'you' plural. Even though it means 'you', it always uses the exact same verb endings as 'they' (ellos or ellas).

Why doesn't temprano have an -s at the end to match the plural ustedes?

Because temprano (early) describes the action of working, not the people. Words that describe verbs are called adverbs, and they never change their form to match gender or number.

Do I need to combine en and el into a single word?