Meaning of the present perfect

Present Perfect, Meaning 1= Something Happened Before Now At An Unspecified Time
(a) Jim has already eaten lunch
(b) Ann hasn't eaten lunch yeg
(c) Have you ever eaten at that restaurant?
The Present Perfect expresses an activity or situation that occured (or did not occur) before now, at some unspecified time in the past
In (a) : Jim's lunch occured before the present timd. The exact time is not mentioned, it is unimportant or unknown. For the speaker, the only important information is that Jim's lunch occured in the past, sometime before now.
(d) Pete has eaten at that  restaurant many times
(e) I have eaten there twice
An activity maybe repeated two, several, or more times before now, at unspecified times in the past, as in (d) and (e)
Present Perfect Meaning 2: A Situation Began In The Past And Continues To The Present
(f) We've been in class since ten o'clock this morning
(g) I have known Ben for ten years. I met him ten years ago. I still know him today. We are friends.
When the present perfect is used with since or for, it expresses situation that began in the past and continue to the present. In (f): Class started at ten. We are still in class now, at the moment of speaking.
Incorrect: We are in class since ten o'clock this morning.