Totally confused when to use demi or moitié

Question

J’ai bu une demi-bouteille.J’ai bu la moitié de la bouteille.Nous n’utilisons qu’une moitie du sac de riz.Il mange qu’une moitie du biscuit.Can someone explain in a different way from the lesson... which basically says they mean the same.

Accepted Answer

Hi Celena,
"Moitié"   is a noun.    "Demi-"    is a prefix
If you go to buy (say) wine, you can buy a half bottle of wine rather than a full bottle.
Je voudrais acheter une demi-bouteille de vin.
You may then decide to drink only half of the contents (la moitié) leaving the rest until another time.
There are a number of further examples but I think that you will see the point between each word in my brief example.
Does this help?
Jim