When do we use the third tense in English with specific verbs in present perfect tense, for example: I have finished, He has worked, We have gone?
You use it when the action happened in the recent past.
"I have eaten" is an action that happened in the more recent past than "I ate".
For example, you could say, "I ate a pizza for my birthday last year."
For something recent, you could say, "OK, I have eaten my sandwich. Now we can go see the movie."
In North America, this tense is not used as much as it is in Britain, which is a pity because it makes a useful statement about when a past action occurred. When it is used in North America, the verb "have" is usually abbreviated. For example, you will more likely hear:
We've run out of time for this project.
You cannot use an indicator of time with this tense. For example, you can't say "I have eaten my sandwich last week."