Sequence of verb tenses

[supratim7]

I keep getting stumped by sequence of verb tenses whenever the first action is in past tense. I thought "once in past, always in past" is the basic rule, but it doesn't seem that simple.

English isn't my first language; sequence of verb tenses and their meaning/grammatical correctness is a weak point. So, please review following examples and evaluate my reasoning/understanding.

I apologize for such a long drawn thread. Much appreciate your help.

PAST + PRESENT

A) In 1550, Greek doctors discovered that blood circulates through veins.
-general truth. "circulates" correct.

B) In 1987, Smith first observed that the Vortex River is full of pollution.
-was polluted then and perhaps still continues to be. "is full" correct.

C) In 2012, I first learned that Gaddafi is a dictator.
-was a dictator then but not alive today. "is" incorrect.

D) Last month, I reduced the number of cigarettes I smoke.
-could have reduced something that happened then, not something that happens now. "smoke" incorrect.

E) Last month, my doctor reduced the number of cigarettes I am allowed to smoke.
-reduced something that continued from then on. "am allowed" correct.

PAST + PAST

F) In 1550, Greek doctors discovered that blood circulated through veins.
-general truth. "circulated" inorrect.

G) In 1987, Smith first observed that the Vortex River was full of pollution.
-was polluted then but perhaps not anymore. "was full" correct.

H) In 1987, anthropologists declared that the many ancient civilizations used complex alloys.
-not sure about the usage/correctness of this construction.

I) In 2012, I first learned that Gaddafi was a dictator.
-was a dictator then but not alive today. "was" correct.

J) Last month, I reduced the number of cigarettes I smoked.
-reduced something that happened then. "smoked" correct.

K) Last month, my doctor reduced the number of cigarettes I was allowed to smoke.
-not sure about the usage/correctness of this construction.

PAST + PAST PERFECT

L) In 1550, Greek doctors discovered that blood had circulated through veins.
-couldn't have discovered something already over. "discovered + had circulated" incorrect.

M) In 1987, Smith first observed that the Vortex River had been full of pollution.
-couldn't have observed something already over. "observed + had been full" incorrect.

N) In 1987, anthropologists declared that the many ancient civilizations had used complex alloys.
-not sure about the usage/correctness of this construction.

O) In 2012, I first learned that Gaddafi had been a dictator.
-not sure about the usage/correctness of this construction.

P) Last month, I reduced the number of cigarettes I had smoked.
-couldn't have reduced something already over. "had smoked" incorrect.

Q) Last month, my doctor reduced the number of cigarettes I had been allowed to smoke.
-couldn't have reduced something already over. "had been allowed" incorrect.

Thank you | Supratim

[RonPurewal]

A) In 1550, Greek doctors discovered that blood circulates through veins.
-general truth. "circulates" correct.

Yes.

B) In 1987, Smith first observed that the Vortex River is full of pollution.
-was polluted then and perhaps still continues to be. "is full" correct.

If you're implying that this condition is going to persist forever -- like the circulation of blood -- then this sentence is ok. If there is any chance whatsoever that the pollution will be gone someday, then use "was".

E.g., My wife didn't realize that my face is ugly until 2009. --> My face is ugly, and will remain that way in perpetuity. (Faces don't spontaneously change, so this sentence is reasonable.)

[RonPurewal]

C) In 2012, I first learned that Gaddafi is a dictator.
-was a dictator then but not alive today. "is" incorrect.

Correct -- If Qaddafi is dead, "is" is wrong.

D) Last month, I reduced the number of cigarettes I smoke.
-could have reduced something that happened then, not something that happens now. "smoke" incorrect.

E) Last month, my doctor reduced the number of cigarettes I am allowed to smoke.
-reduced something that continued from then on. "am allowed" correct.

Yes.

By the way -- note that "has/have reduced" is seen from a PRESENT point of view. So, with that kind of verb, you'd want the present: I have reduced the number of cigarettes that I smoke.

[RonPurewal]

F) In 1550, Greek doctors discovered that blood circulated through veins.
-general truth. "circulated" inorrect.

In general, yeah, you wouldn't want this.

There could be situations in which it'd make sense. For instance, say a Martian came to Earth, and noticed that the sun came up in the east for the first ten days he/she/it was here.
At this point the Martian would likely not yet be convinced that this would happen forever and ever. So, the Martian noticed that the sun consistently came up in the east would convey that idea better. With "comes" you'd be implying that the Martian had figured it out in general.

[RonPurewal]

G) In 1987, Smith first observed that the Vortex River was full of pollution.
-was polluted then but perhaps not anymore. "was full" correct.

Yeah. Or even if the river is still polluted now, as long as there's some reasonable chance that it will be cleaned up.

By the way, there's absolutely no way that GMAC would test anything this subtle on the exam. If you see a difference like this one, then go find the easy thing they're distracting you from. (:

[RonPurewal]

H) In 1987, anthropologists declared that the many ancient civilizations used complex alloys.
-not sure about the usage/correctness of this construction.

The sentence is fine.

"The" in front of "many civilizations" implies that every ancient civilization used alloys. (The ten survivors of the plane crash sued the airline tells us two things: first, that there were exactly ten survivors, and, second, that all ten of them sued the airline.)
GMAC has never written a question that explicitly tests a/an/the -- and likely never will (would discriminate strongly against speakers of some languages and strongly in favor of those of others) but I guess I'm just throwing it out there, since the rest of the sentence is fine.

I) In 2012, I first learned that Gaddafi was a dictator.
-was a dictator then but not alive today. "was" correct.

yeah, this one is fine.

J) Last month, I reduced the number of cigarettes I smoked.
-reduced something that happened then. "smoked" correct.

fine, as noted above.

[RonPurewal]

K) Last month, my doctor reduced the number of cigarettes I was allowed to smoke.
-not sure about the usage/correctness of this construction.

If the reduced number is not the same number today, then this sentence is correct. (E.g., Last month the doc reduced the number of cigarettes that I [i]was allowed to smoke -- and just yesterday she ordered me to quit smoking altogether.[/i])

[RonPurewal]

L) In 1550, Greek doctors discovered that blood had circulated through veins.
-couldn't have discovered something already over. "discovered + had circulated" incorrect.[/quote]

Hopefully, this comment is intended to apply only to the circulation of blood, for which that tense is, indeed, nonsense.

in general, of course they could discover something that had already happened! In fact, "discovering something that has/had already happened" is the nature of pretty much all evidence. In fact, that's not a bad definition of "evidence" -- things that let you discover an already-completed action.
By looking at paw prints, they discovered that wolves had run through their camps.
No one was able to determine exactly what had killed Caesar.

[RonPurewal]

M) In 1987, Smith first observed that the Vortex River had been full of pollution.
-couldn't have observed something already over. "observed + had been full" incorrect.

This doesn't work with "observed", since "observed" is limited to direct observation. If you replace "observed" with a verb that can describe a more indirect process of using evidence -- AND supplied a timeframe to which "had been full" could apply -- then this could work.

In 1987, by studying isotopes in the rock bed beneath the river, scientists discovered that the river had been polluted before humans settled in the area.

[RonPurewal]

N) In 1987, anthropologists declared that the many ancient civilizations had used complex alloys.
-not sure about the usage/correctness of this construction.

Doesn't work unless (at least) one of two things exists:
1/ Some sort of direct relevance to the situation at hand;
2/ Another past event to provide a timeframe/point of view.

E.g.,
#1 --> Health agencies were concerned that the alloys could make people sick, but anthropologists pointed out that many ancient civilizations had used those same alloys without incident.
#2 --> Anthropologists found evidence that many ancient civilizations had used these alloys before the date at which they were originally thought to have been discovered.

O) In 2012, I first learned that Gaddafi had been a dictator.
-not sure about the usage/correctness of this construction.

Same deal, you'd need #1 or #2 above.

P) Last month, I reduced the number of cigarettes I had smoked.
-couldn't have reduced something already over. "had smoked" incorrect.

Yes.

Q) Last month, my doctor reduced the number of cigarettes I had been allowed to smoke.
-couldn't have reduced something already over. "had been allowed" incorrect.

Yes.