Learn Italian: Serve un pass per entrare in piazza oggi. - You need a pass to enter the square today.

Servire here means "to be necessary". The thing that is needed (un pass) is the grammatical subject, which often follows the verb for emphasis. It works much like piacere: literally, "A pass is necessary." If you wanted to use "you need", you would say hai bisogno di un pass, but serve un pass is a very natural, impersonal alternative.

Since the verb is servire, why do we use per entrare instead of ad entrare?

While servire a is used to describe a tool's purpose (e.g., questo coltello serve a tagliare meaning "this knife is used to cut"), here we are expressing a condition or goal. To say "A pass is needed in order to enter," you use per + infinitive.

Why is it in piazza instead of alla piazza or nella piazza?

Certain common locations (like piazza, banca, chiesa) take the simple preposition in without a definite article when referring to them generally. You only use an articulated preposition like nella if you add specific details, such as nella piazza del Duomo.

How do English loanwords like pass work with gender and plurals in Italian?