Forma di cortesia

The use of courtesy forms is very frequent in Italian language. When you need to address people - strangers, teachers, professional superiors, distant acquaintances, and anyone about whom you’re unsure - formally, you have to use the pronoun Lei (feminine, 3rd person singular, for one person, both male and female), with the verb in the 3rd person singular. The capitalisation of Lei to distinguish it from lei ‘she’, is becoming less and less common, even in formal correspondence. In contemporary spoken Italian, it is also extremely rare to make adjectives, past participles, etc., agree in gender with Lei if the subject is masculine.

Scusi, Lei è il nuovo insegnante? - Excuse me Sir, are you the new professor?
Lei, dottore, ha sempre ragione! - You, Mr doctor, are always right!

The plural form to address people formally is Voi, with the verb in the 2nd person plural.
Buonasera, Voi siete i Signori Rossi, immagino. - Good evening, you are Mr and Mrs Rossi, I guess.

In very formal contexts, the plural form can be Loro, with the verb in the 3rd person plural.
Buonasera, che cosa prendono i Signori? - Good evening, what would you like? (lit. Good evening, what do the Sirs desire?)

Courtesy form
Singular Lei - with the verb in the 3rd person singular
Plural Voi - with the verb in the 2nd person plural
Loro (rare) - with the verb in the 3rd person plural