When in doubt about whether to use tu or Lei, non-native speakers are encouraged to use the more formal Lei, leaving it to the native speaker to propose the use of tu (frequently by the phrase, diamoci del tu! = Let’s use the tu form!). If a student or an adult uses tu inappropriately, it could cause offense to the addressee.
Italians may also encourage (foreign) students to use the Lei form with an expression like the following:
Mi dia del Lei = Let’s use the Lei form.
Following are some appropriate uses of Lei:
Grazie, Lei è molto gentile.
Thank you, you are very kind.
Dottore, l’ho vista ieri in piazza.
Doctor, I saw you yesterday in the piazza.
*As you will have noticed by now, Lei is often written with a capital L (especially in formal correspondence), as is its corresponding direct- and indirect-object pronouns (always in the feminine form: La, Le). However, it should be noted that the trend in current usage is toward a lowercase l, e.g., “Come si chiama lei?”