Learn Italian: Scusi, signore, apre la porta? - Excuse me, sir, will you open the door?

The speaker is being polite and using the formal "you" (Lei). For regular -ire verbs like aprire (to open), the formal "you" takes the same ending as the "he/she" form, which is -e. If you were talking to a friend, you would use the informal "you" ending and say apri.

The English translation says "will you open", but isn't apre in the present tense?

Exactly! Apre is the present tense ("you open" or "you are opening"). In Italian, the present tense is frequently used for immediate future actions or polite requests. So asking "apre la porta?" right now naturally translates to "will you open the door?" in English.

Why use scusi here instead of scusa?

Scusi is the formal way to say "excuse me". Since the speaker is addressing a stranger politely (calling him signore), they must use the formal version to match. You would use scusa with friends, family, or children.

I've seen "Mr." or "sir" written as signor. Why is it signore here?