Metà o Mezzo?

Some students of mine, when talking about their nationality, tell me that they are "half and half", for instance half English and half French. They say: sono mezzo/a inglese e mezzo/a francese. I correct them and say: sono metà inglese e metà francese. It would sound even better to add a preposition and say: sono per metà inglese e per metà francese.

Sometimes metà and mezzo can be used interchangeably, but always bear in mind that metà is an accented word and is unchanged, whereas mezzo does change gender and number as necessary. Here are some sentences that show you various uses of both words. In the last example both are possible (to some extend):

Le posso fare lo sconto della metà del prezzo
I can give you a discount of half the price

Arrivati a metà strada, si è rotta la macchina
Arrived half-way through, the car broke down

Arriviamo tra mezz’ora
We’ll arrive in half an hour

È mezzo matto
He is half mad (he is a bit crazy)

a. Vuoi tutta la mela? Do you want the whole apple?
b. No, solo la metà No, only half
c. No, solo mezza No, only half (more colloquial)

Notice that "mid" tends to be translated as "metà":
metà marzo = mid March