Learn French: Ils nettoient le bureau du comptable. - They are cleaning the accountant's office.

The verb is nettoyer (to clean). For verbs ending in -oyer, the y must change to an i in the present tense whenever the ending is silent. Because the ils ending (-ent) is silent, it becomes ils nettoient.

How do you pronounce nettoient? Do I pronounce the -ent ending?

No, you do not pronounce the -ent. For verbs conjugated with ils or elles, the -ent ending is always completely silent. You simply pronounce the word like "neh-twa".

Why do we use du before comptable instead of "de le"?

Du is a mandatory contraction. In French, when the word de (of) meets the masculine word le (the), they must merge to form du. It literally translates to cleaning the office "of the" accountant.

Does bureau mean "office" or "desk"?

It can mean both! Un bureau can refer to the physical desk you work at, or the entire room. In this sentence, they could be cleaning the accountant's physical desk or the whole office space, depending on the context.