Is it more informal to say "festetek" than "festek"?
It makes the subject of the sentence different!
festetek = fest + etek = "you (plural, informal) paint"
festek = fest + ek = "I paint"
Nagyon szépen köszönöm!
Festetek is in past tense. Te correct sentence would be: Milyen házat festesz?
No, it is not past tense. Festetek is the second person plural (present tense).
Every past-tense form of fest begins with festett-. There are more t's needed.