"Sei una donna."
This does not accept Lady as a translation for donna. I was under the impression that donna could be translated as woman or lady.
The hover translation suggests Sei means "are", with you are as suggested use.
Since there was not "you" in the sentence, I picked one of the other meanings: Six one woman.
You don't always need an explicit subject in Italian, based upon the conjugation of the verb, the subject can be implied.
Because Italian doesn't always need an explicit subject. Technically, "(Tu) sei una donna" is grammatically correct, but it is less used, and you seldom hear it in actual conversations.
They're homographs (different words that happen to be written the same). - "sei" can be "six" - "sei" can also be "are", as in "you are" ("io sono, tu sei, etc." = "I am, you are, etc."). Funny note: In the very hypothetic case that you wanted to tell a woman: "You are six women (in one)"; you would say "[tu] sei sei donne (in una)".