"The birds ate all the grain."
Translation:La birdoj manĝis la tutan grenon.
No, because "ĉion" means "everything" and you can't say "everything grain". It's a pronoun (that stands on its own), not an adjective (that can be before a noun).
I don't think "ĉiun grenon" would work, either, because "greno" is a mass noun... it would be like saying "the birds ate every rice and drank every water". You could say "ĉiun greneron" (every individual grain).
manĝi is "eat"; formanĝi is literally something like "eat away" -- it implies more strongly that after the eating is finished, the food is all gone.
It's a bit like an aspect distinction, as with German essen versus aufessen: one is neutral, the other more perfective, implying a completion.