Learn Dutch: Hij bereidt zich voor op het werk.

The verb here is voorbereiden (to prepare). It is a separable verb. In a normal present-tense sentence, the main part of the verb (bereidt) takes the second position, and the prefix (voor) gets pushed to the end of the clause.

Why does the sentence use the word zich?

In Dutch, the verb to prepare is reflexive: zich voorbereiden. It literally translates to preparing oneself. Since the subject is hij (he), we use the reflexive pronoun zich (himself).

The English translation says "for the work", so why does the Dutch sentence use op instead of voor?

Many Dutch verbs have fixed prepositions that you just have to memorize because they don't map perfectly to English. To say you are preparing "for" something, you must use zich voorbereiden op.

How would this sentence look if we added a modal verb like moet (must)?

If you add a modal verb, the modal takes the second position in the sentence. The separable verb is pushed to the very end and glues back together into its full infinitive form. It becomes: Hij moet zich op het werk voorbereiden.