Learn Italian: Sta aprendo la bombola prima di immergersi. - He is opening the tank before diving.

When you say "before doing [something]" in Italian, you must use the phrase prima di followed by the infinitive form of the verb. Even though English uses an "-ing" word ("before diving"), Italian always uses the infinitive after a preposition.

Why is the si attached to the end of immergersi?

Immergersi is a reflexive verb (literally "to submerge oneself"). When an infinitive verb takes a reflexive pronoun, you drop the final -e of the infinitive and attach the pronoun directly to the end. So, immergere + si becomes immergersi.

Could I use prima di immergersi if he was opening the tank for someone else to dive?

No! You can only use prima di + infinitive when the subject of both actions is the exact same person (he is opening the tank, and he is diving). If the subjects were different, you would have to use a different grammatical structure (prima che).

Why does the sentence use sta aprendo instead of just apre?

Sta aprendo is the present continuous tense, formed with the verb stare plus the gerund (aprendo). Just like the English "is opening," it emphasizes that the action is happening right at this very moment. Apre ("opens") is also grammatically correct, but less focused on the ongoing, immediate action.