Incidentally if you want more grammar terms:
ate is the perfect tense of to eat,
it is an irregular verb in the sense that it doesnt use the ---ed morpheme that most verbs do, walked, smiled, etc.
eaten is know as the past participle, and again it is irregular, although that ---en morpheme is more common in participles.
I had walked,
in this case you can see that the perfect tense form and the past participle form of to walk are the same.
If you look at many verbs you will find no difference between the perfect tense and the participle form, smiled, had smiled; danced; had danced; killed; had killed
to eat has difference
I ate and
I had eaten
If you are a native English speaker you will know the rest,
try with :
"to choose"
I chose
I had ........?
to drive
I drove
I had ....?
to hit
I hit
I had ...? (hahah a trick one)
to fly
I .... ?
I had .....?
hope this helps, there are lots of websites which explain this sort of stuff - if you look around