Learn Farsi: فردا میرین بازار؟ - Are you all going to the market tomorrow?

Just like in English ("Are you going to the market tomorrow?"), Farsi frequently uses the present tense to talk about fixed plans in the near future. While Farsi does have a specific future tense, it sounds very formal and is rarely used in conversation. For everyday spoken Farsi, the present tense is the natural choice.

I learned the verb "you (plural) go" as می‌روید (mi-ravid). How does it become میرین (mirin)?

میرین is the casual, spoken version of the formal می‌روید. In everyday conversational Farsi, the present stem رو (rav) shrinks to make it faster to say, and the plural "you" verb ending ـید (-id) almost always shifts to ـین (-in).

Shouldn't there be a preposition like به (to) before بازار?

In formal written Farsi, yes, it would normally be به بازار (to the market). However, in everyday spoken Farsi, the preposition for a destination is almost always dropped when you use verbs of motion like going, coming, or arriving.

Since there is no question word like آیا (āyā) here, how do we know this is a question?

In everyday spoken Farsi, the formal question marker آیا is almost never used. Instead, you turn a statement into a question simply by raising your voice pitch at the very end of the sentence, specifically on the last syllable of بازار.