Learn Farsi: برگزارکننده باید پول را پس می‌داد. - The organizer should have refunded the money.

It uses the rule you are learning right now: the unchanging word باید (bāyad, meaning should or must) plus the past continuous tense of the verb. Here, پس می‌داد is the past continuous of پس دادن (to give back or refund). Combining باید with the past continuous creates the meaning that someone 'should have done' something.

Why does the prefix می go in the middle of the verb پس می‌داد?

پس دادن is a compound verb, which means it is made of two parts. In Farsi, when you use the continuous prefix می with a compound verb, it always sits between the non-verbal part (پس) and the conjugated verb part (داد).

How is the word برگزارکننده (organizer) built?

It comes from the compound verb برگزار کردن (to organize or hold an event). The suffix کننده means 'doer' and is used to turn a verb into a noun for a person who does that action. Attached to برگزار, it creates the word for the person organizing the event.

Why is را used after پول (money)?

را is the definite object marker. The sentence is talking about a specific amount of money (the refund for the event), rather than just money in general. Because پول is the direct object that should be given back, we use را to show it translates to 'the money'.