Learn Polish: Dlaczego wczoraj późno wstałaś? - Why did you get up late yesterday?

The verb wstałaś comes from wstać (to get up). In the past tense, the -łaś ending specifically means "you" (singular) did it, and that "you" is female. If you were asking a male, the ending changes to -łeś, making it wstałeś.

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

Polish usually drops subject pronouns like ty (you) because the verb ending gives it away. The -ś at the very end of wstałaś tells us it means "you". You could say Dlaczego ty wczoraj późno wstałaś?, but that would sound like you are heavily emphasizing the "you".

Why does the word for "late" end in an -o here?

Because it is an adverb describing how the action was done. Późno means "late" as in acting late. If you were describing a noun (like a "late morning"), you would use the adjective form późny (or późna or późne depending on the noun's gender).

Is the word order strict? Could I say Dlaczego wstałaś wczoraj późno?

Yes, you absolutely can! Polish word order is very flexible. Both versions are natural, but placing the verb at the very end, as in Dlaczego wczoraj późno wstałaś?, is extremely common and just puts a little extra focus on the action of getting up.