Learn Farsi: آیا پختن گوشت یک مشکل است؟

To get the past stem, you simply take the infinitive form پختن (pokhtan) and drop the final ن (n). This leaves you with the past stem پخت (pokht). You will use this base to build past tense verbs.

Why is the infinitive پختن (to cook) used here to mean "cooking"?

In Farsi, the infinitive form of a verb can act just like a noun. It works exactly the same way as "-ing" words (gerunds) do in English when talking about an activity. So here, پختن functions as the subject "cooking".

Why is the verb است (is) at the very end of the sentence when the English translation has it at the beginning?

Farsi follows a Subject-Object-Verb word order. This means the main verb of the sentence almost always goes at the very end. Even though the question word آیا starts the sentence, the main verb است must wait until the end.

Do I always have to use آیا to ask a yes/no question?

No, you do not. While آیا clearly marks a formal yes/no question, in everyday spoken Farsi, people usually drop it. Instead, they just keep the regular sentence structure and raise the pitch of their voice at the end to show they are asking a question.