Il ne mange qu'une moitié du biscuit. He only eats one half of the biscuit.

Question

Does this not also means 'he eats only one half of the biscuit'? Or if not, how do you differentiate between these two meanings? Thanks

Answer

Bonjour Nick,
Let's see if this helps any.
Toute la journée, il n'a mangé que la moitié d’un biscuit 
All day, he ate only one-half of a biscuit
compare the above to this phrase:
Il n'a mangé que la moitié du biscuit
He ate only half of the biscuit
In other words, the differentiation comes whether is un biscuit, a biscuit, or du biscuit, remember that «du» is the shortened form of «de le» hence the biscuit.
J'espère que cela vous aidera.
Bonne chance,
Ron