Learn Arabic (Modern Standard): المكتب نظيف.

In Arabic, there is no word for "is" or "are" in the present tense. By simply placing a definite subject like المكتب (the office) right next to a descriptive word like نظيف (clean), you automatically create a complete sentence meaning "The office is clean."

If I want to replace "The office" with a pronoun to say "It is clean", which one do I use?

You would use the pronoun هو (huwa). In Arabic, all nouns are either masculine or feminine. Because مكتب is a masculine word, you use هو, which means both "he" and "it" (for masculine things). To say "It is clean," you would just say هو نظيف.

Why doesn't the word for clean (نظيف) have "al-" (the) attached to it?

In an Arabic "is" sentence, the subject usually has "al-" (ال) to make it specific, but the descriptive word does not. If you put "al-" on both words (المكتب النظيف), they link together to mean "the clean office" (a phrase) rather than "The office is clean" (a complete sentence).

Can the word المكتب mean anything else besides "the office"?

Yes! المكتب can mean "the office" or "the desk" depending on the context. For this sentence, "The desk is clean" would also be a perfectly correct translation.