Learn Farsi: لازم است که در این محیط از ماسک استفاده کنی. - It is necessary that you use a mask in this environment.

Because of the phrase lâzem ast ke (it is necessary that). Impersonal phrases that show necessity or possibility require the verb that follows them to be in the present subjunctive form. Here, the indicative mikoni becomes the subjunctive koni.

Why is the preposition az (from) placed before the word mask?

The verb for "to use" is estefâdeh kardan, and it always pairs with the preposition az. In Farsi, you do not use something directly; you "make use from" it. That is why the sentence says az mâsk estefâdeh koni.

Where is the be- prefix for the subjunctive verb? Shouldn't it be bekoni?

For compound verbs like estefâdeh kardan, it is very common to drop the be- prefix on the "doing" part of the verb (kardan). While estefâdeh bekoni is not strictly wrong, estefâdeh koni is much more natural and standard in spoken and written Farsi.

Do I always have to include ke after lâzem ast?

Not necessarily. Just like you can drop "that" in English (It is necessary you use...), Farsi speakers frequently drop ke in everyday speech.