"Tłʼoh chin yį́yą́."
Translation:He is eating an onion.
The non-progressive present "eats" should be accepted as well as the progressive present "eating" in the English translation, as the progressive aspect hasn't even been taught yet imho.
Addendum: As far as I can tell, Navajo doesn't have any definite or indefinite articles, so the translation should accept "the onion" as well as "an onion" imo.
I am by no means an expert at Navajo, and am not a native speaker. I am however trying to learn the grammar as best as I can.