Learn Arabic (Modern Standard): أنت في بيت نظيف.

In written Arabic without short vowel marks, أنت can be read as anta (when speaking to a man) or anti (when speaking to a woman). The spelling is exactly the same! Since this sentence does not give us extra context about who is being spoken to, you can use either one.

Where is the word for "are" in this sentence?

Arabic does not use a present tense word for "to be" (like am, is, or are). You simply place the subject pronoun أنت (you) right next to the phrase في بيت نظيف (in a clean house). The word "are" is automatically understood!

Why is the word for clean (نظيف) placed after the word for house (بيت)?

In Arabic, describing words (adjectives) always come directly after the noun they describe. So instead of saying "clean house" like in English, the structure is literally "house clean" (بيت نظيف).

There is no word for "a" here. How do we know it means "a clean house" and not "the clean house"?

Arabic does not have a separate word for "a" or "an". A noun without the prefix ال (the) is usually indefinite, meaning it translates with "a" or "an" in English. Notice that the adjective نظيف also drops the ال to perfectly match the noun it describes.