"He starts to eat."
Translation:Dia mulai makan.
Memulai would simply shift the emphasis of the action from the object to the subject. Hence, who is beginning to eat rather that what the person is beginning to do. I just thought of a mnemonic to help remember this ... the prefix, ME- makes the sentence about 'me'.
I appreciate the explanation, it's great to know what you told me, but since there are not contexts I don't see any reason why "memulai" can't be accepted for this sentence...