OG #21 (verbal review purple book)

It is possible that Native Americans have migrated to the Western Hemisphere over a bridge of land that once existed between Siberia and Alaska.

OG criticizes the use of the present perfect. It recommends the use of the simple past. I’m OK with that for the sentence above because the Native American migration has clearly stopped.

But take the following:

(1) It is possible that the disease has spread through food contamination.
(2) It is possible that the disease spread through food contamination.

In (1) the disease still persists.
In (2) we don't know if the disease still persists.
But both look OK to me, unless I am missing something.

(1) It is possible that he has committed the murder.
(2) It is possible that he committed the murder
Same question. Pick (1) or (2) ? Both look OK.

An additional confusion arises since the present perfect can be used for an action that completed at an indefinite time in the past.
I have attended college.
[I attend no more].

Here its the opposite. The present perfect seems correct even thought the action has ceased.


I am sorry if I have confused everyone, but please help me in choosing b/w simple past and present perfect.

Ah yes. You're running head-on into some of the great subtleties of the English language**. In most of the cases you're describing, either tense is fine, in terms of grammar - but your meaning will dictate, one way or the other, the correct way to write the sentence.

I'll address the LAST one first: "I have attended college."
You would use the present perfect here if your college attendance is still relevant to the current situation (thus 'continuing into the present' at least in terms of importance, if not actual occurrence). For example, if your bosses at work want to know who attended graduate school, for the purposes of awarding a new position to someone with a graduate degree, you would most likely say "I've attended graduate school." On the other hand, if you have a degree from, say, a foreign country that carries no weight here in the States, you might say something like "I attended graduate school in country X" (the implication being that it doesn't matter anymore).

About the murders, yes, either is OK. Your choice would be a subtle one - and would be based in some way upon the recency of the murder and/or its immediate effect upon the current situation - but would probably flow naturally from your lips (if you're a native speaker of English). In any case, the GMAT is not going to catch you out on a line as thin as this one.

You're right on about the diseases.

**It seems that pretty much all other languages are this subtle, too.