Learn Portuguese (Portugal): Enquanto o homem culpado não disser a verdade, tem de ficar na prisão. - While the guilty man doesn't tell the truth, he has to stay in prison.

disser is the Future Subjunctive of dizer ("to say" or "to tell"). For highly irregular verbs like this, the Future Subjunctive is built using the "eles/elas" form of the simple past (Preterite). Because "they said" is disseram, we drop the -am to get the base disser-. Then we add the Future Subjunctive endings, which for "ele" is nothing, leaving us with disser.

Doesn't enquanto usually just mean "while" and take a normal present tense verb?

It does when it describes things happening at the same time in the present or past (like "While I eat, I read"). However, when enquanto points to an unknown duration in the future—translating more like "as long as"—it acts as a trigger for the Future Subjunctive. We use não disser because we don't know when, or if, he will finally tell the truth.

Why use tem de ficar instead of just deve ficar or precisa de ficar?

Ter de followed by an infinitive is the most natural and common way to say "have to" or express a strong obligation in European Portuguese. While deve ("must" or "should") or precisa de ("needs to") could also make sense, tem de is the standard way to express this kind of hard rule or requirement.

Could ficar just be translated as "be" here?