It's a very strange verb, because it doesn't have an infinitive form, and is the only (I think) verb that shows the gender of the subject of the sentence in the Present Tense. Usually "powinien" (3rd person singular masculine) is used instead of the infinitive, for example in the dictionaries.
Therefore "powinnam" will be said by a woman and "powinienem" will be said by a man.
Moreover, this can mean either Present Tense or Past Tense. Well, in theory it could be even Future Tense, but that's not very common. Many people however use the plusquamperfect form and say "powinnam była"/"powinienem był" to make it obvious it's Past Tense and I'd definitely recommend it.