tense - Can we use 'ago' with past perfect?

Where is the error?

For the recent exhibition, the museum ① acquired a rock ② from a volcano that ③ has erupted ④ more than two thousand years ago.

The answer is '③ has erupted'. Yeah, that is easy. We can't use present perfect with 'ago'.

But can we use past perfect with 'ago'?

A. For the recent exhibition, the museum acquired a rock from a volcano that erupted more than two thousand years ago.
B. For the recent exhibition, the museum acquired a rock from a volcano that had erupted more than two thousand years ago.

Some (we are not native-speakers) say you can't use 'ago' with past perfect, so you must use 'erupted' in this sentence.
Some say no, no, you can't use 'ago' with present perfect, but you can use 'ago' with past perfect because both are past tense. So,'volcano erupted' happened first than 'acquired rock', therefore you must use 'had erupted'.

Question

1. I know present perfect is not used with 'ago'. But what about 'past perfect'?



2. Can we use 'ago' with past perfect (although usually used with 'before')? And which is correct between A and B?

Answer

There's nothing special about "ago" in particular. "Ago" just clarifies that the action took place in the past.

For example, the present perfect is equally incorrect in this sentence:

The museum acquired a rock from a volcano that has erupted in 1991.

"Ago" is not used here, but the present perfect is still incorrect, because the eruption took place in the past.

So, don't get hung up on "ago". Either the simple past or the past perfect could be correct, but the rules for doing so are the same as the regular rules for using the simple past vs. the past perfect.