Hello. I can help you, but I can't see the lesson section page 3. I'm Cuban and a Spanish teacher. So, if you give me more details, I can definitely help you.
¿Tú subiste por la escalera o en el ascensor?
Why subiste?
Subeste, nope, nope, no, this isn't a word in Spanish. This is how we know this one is wrong.
Subaste? Not a word either. This is crossed out.
Subes, This is a word but it's stating something instead of asking a question. "Tú subes por las escaleras = You climb/ go up the stairs".
So subiste is the right one!
Hope this helps!~
Los idimas se hablan en España es catalán y castellano
