"He starts to eat."

Translation: Dia mulai makan.

[boringtomi]

Why can't "memulai" be accepted here?

[lockers001]

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'.

[boringtomi]

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...

[lockers001]

Definitely

[duolingotfek]

'Memulai' would need an object.

Meanwhile, 'mulai makan' is verb (start eating)