OG Verbal Review - SC - #21

[Guest]

In the strategy guide, it says that if two actions take place in the past, the action that took place before the other action needs to be in past perfect tense.
My problem is I don't always understand what an action constitutes.

e.g. Teacher thought that Jimmy had cheated in the exam (here two actions are "thought", which happened after Jimmy "cheated" and hence cheated is in past perfect tense).
But in this example (Verbal Review, #21):
It is possible that Native Americans originally "have migrated to the Western Hemisphere over a bridge of land that one existed" between Siberia and Alaska
are there two actions ("existed" and "migrated")? The correct answer here uses both simple past tense.

So I don't really understand when to think of two verbs as actions.

Any tips?

[Guest]

Figured I should put answer choices here as well for easy reference:

Verbal Review, #21
It is possible that Native Americans originally have migrated to the Western Hemisphere over a bridge of land that one existed between Siberia and Alaska

(B) were migrating to the Western Hemisphere over a bridge of land that existed once
(C) migrated over a bridge of land to the Western Hemisphere that once existed
(D) migrated to the Western Hemisphere over a bridge of land that once existed
(E) were migrating to the Western Hemisphere over a bridge of land existing once

Now if there were an answer choice that looked something like "migrated to the Western Hemisphere over a bridge of land that had existed", be a better choice?

[yamini]

here choice D is correct. I am adding another small quesion.

migrated to the Western Hemisphere over a bridge of land that once existed .

But why can't we use like

migrated to the Western Hemisphere over a bridge of land that WAS once existed.

[Guest]

Yes D is the correct answer but that's not the intention of the question. I'd like to understand when we can think of two verbs as two actions and use a past perfect tense.
Anyways, about your query: I think that verb would then become "was existed" and that would be incorrect in any tense.

[StaceyKoprince]

For past perfect, the two events have to take place at different times. If they migrated over the bridge, they had to do it when it existed... therefore, the two events take place at the same time. If they take place at the same time, you use the same tense (in this case, simple past). Past perfect can ONLY be used if the two events take place at DIFFERENT times.

"Was existed" is never correct construction.