You certainly can form questions in English without the use of a pronoun: 'got a pen?' is one simple example. It may be a colloquialism to phrase a question in this way, but it is not so informal as to be considered slang. Whether or not Duolingo accepts colloquialisms is a different issue.
In English you usually need the verb "do" at the beginning of a sentence if it's a question. If you say "remember your sister" in Enlgish, it is imperative (pamiętaj twoją siostrę).
Unfortunately, you can't form a question by just adding a question mark or adding intonation in English.
Hope that helps :)