What is past perfect tense of eat?

Answer 1

That depends on who or what you're talking about.

Jeff was a cannibal, he had eaten his brother.

Jeff's brother had been eaten.

I don't know why I felt the need to refer to cannibalism in order to provide this answer. I am not a cannibal, just in case you're wondering.

Answer 2

"Had been eating" is the past perfect progressive tense.