Hi Kathryn,

"Il a mangé la moitié de son pain"   --> He has eaten half of his bread   --  There is no restriction in this sentence.  There remains the possibility that he may finish off the rest of the bread in due course?

"Il n'a mangé qu'une moitié du biscuit"  Here there is a restriction  -- He has eaten only half of the biscuit. The sense here is that he has eaten all that he wants of the biscuit and no more.

To my mind, this is the subtlety between the two sentences.

Hope this helps.

Bonne journée

Jim

Same difference as in English: he ate one half of... vs. he ate half of...