Learn Farsi: عکس گرفتن در موزه آزاد است. - Taking photos in the museum is allowed.

gereftan is the infinitive form of the verb, which you can recognize by the -tan ending. In Farsi, you can use the full infinitive just like a noun to talk about the activity itself. Here, aks gereftan acts like the English action word taking photos.

How do I find the present stem of gereftan if I want to say that I take a photo?

If you drop the -an from gereftan, you get the past stem gereft. However, the present stem for this verb is irregular, so it does not look like the infinitive! The present stem is gir, which you must memorize to build present tense verbs like migiram (I take).

Does the word aks change if we are talking about taking multiple photos?

No. In compound verbs like aks gereftan (to take a photo or to take photos), the noun part usually stays in its singular form, even when the English translation uses a plural word.

I learned that azad means free. Why does it translate to allowed in this sentence?

In Farsi, when you say an action is azad in a public space like a museum, it means the action is unrestricted or permitted. You can think of it as being free to do that activity.