Learn French: Oui, tu es sympa et très poli. - Yes, you are friendly and very polite.

Tu is informal, used for friends, family, or kids. You can still tell a friend they have good manners! If you were talking to a stranger, you would use the formal vous and say, Oui, vous êtes sympa et très poli.

Is sympa a slang word? It looks like an abbreviation.

It is a very common everyday abbreviation for sympathique, which means nice, friendly, or pleasant. French speakers use sympa all the time in casual conversation.

How do we know if the tu in this sentence is male or female?

Look at the word poli! Because it doesn't have an extra e at the end, we know it is describing a male. If the speaker were talking to a female friend, they would spell it très polie.

Why doesn't sympa change to match the masculine tu like poli does?

The full word sympathique already ends in an e, so it is exactly the same for both masculine and feminine. The short version sympa also stays the same whether you are talking to a man or a woman.