The use of 'Lei'(you) instead of 'tu'(you)

In Italian, if you are speaking to a person you do not know very well or who is you "superior", say your professor at university or your supervisor at work or your boss, you should not address them using the second person singular 'tu' but you ought to use what is informally known as 'il lei'. As a consequence, the verb concords with the subject 'Lei' and when you refer to this person using 'il lei' you must make sure that the verb agrees with the thrid person singular.

eg:

ENGLISH: Could you repeat, please?

ITALIAN: Può ripetere, per favore? (The subject 'lei' is omitted, but is contained in the verb)

Instead of: Puoi ripetere, per favore?

No matter if the person address is male or female, you'll still need to use 'il Lei', for example:

ENGLISH: Sir, I was looking for you.

ITALIAN: Signore, la stavo cercando.

Instead of: Signore, ti stavo cercando.