Learn French: À supposer que le procureur dise la vérité, l'accusé ira en prison. - Supposing that the prosecutor tells the truth, the accused will go to prison.

The phrase à supposer que (supposing that / assuming that) sets up a hypothesis or a condition that is not confirmed to be true. Expressions that introduce a hypothetical scenario in this way require the subjunctive mood. Here, dise is the present subjunctive of the verb dire (to say/tell).

Why do we use the future tense (ira) in the second part of the sentence?

When you make a hypothesis about the present or future, the consequence is often in the future tense (le futur simple). Even though the first part uses the subjunctive because of à supposer que, the sequence of tenses follows the same logic as a typical if-then statement: Assuming X is true now, Y will happen later.

How do I remember the subjunctive form dise?

Be careful not to confuse it with the indicative! The present indicative for il/elle is dit (as in il dit). The subjunctive is built from the ils/elles present indicative stem (ils disent). You drop the -ent and add the subjunctive -e ending, which gives us il dise.

Could I use en admettant que in this sentence instead?

Yes, absolutely! En admettant que (admitting that / assuming that) is very similar in meaning to à supposer que and also requires the subjunctive. You could say En admettant que le procureur dise la vérité... with almost no change in meaning.