过去完成时的使用

绿杨小庙

过去完成时真的很头疼呀，发一条prep的题目，尽管很多人各种解释，我觉得都有马后炮的成分，当我遇到新的题目还是不会选过去完成时，先放在这里：
Ozone, a special form of oxygen that screens out harmful ultraviolet rays, reaches high concentrations twelve miles above Earth, where it has long appeared that it was immune from human influence; we have now realized, though, that emissions of industrial chlorofluorocarbons deplete the ozone layer.

• Ahas long appeared that it was immune from
• Bhas long appeared to have been immune from
• Chas long appeared as being immune to
• Dhad long appeared immune to
• Ehad long appeared that it was immune to
  

我的答案：B

正确答案：D

尽管说realize理解成过去的动作，所以用过去完成时，但是这里是have realized，不是过去的动作呀，而且分号前后是两个独立的句子，前面一个句子的时态应该根据前面句子的内容决定，怎么扯到后面的句子了？不理解，不想纠结，先放着，多攒一点这样变态的题目。

AMBER513

note that the past perfect can be used, roughly speaking, in two different circumstances:
1) for past actions that were ongoing until, or relevant to, a later past action (this is what you're thinking about);
OR
2) for past actions that were ongoing until, or relevant to, a later past TIME MARKER.

for #2, the second time marker could be something explicitly mentioned in the sentence.
before the introduction of soft landing mats, high jumpers had been forced to use the western-roll style of jumping
--> note that there is no second verb, but "the introduction of soft landing mats" is a time marker.

however, if the context of the sentence clearly IMPLIES a second time marker, then that second time marker may not even have to be mentioned explicitly.
in this sentence, the second time marker is implied by the phrase "earlier in the year" -- i.e., these fears were ongoing until a certain point that is already in the past.

--

as a final note, make sure you know that the difficulty of learning verb tenses -- not just in english but in any language -- is going to be vastly higher than the difficulty of learning almost anything else in a language.
verb tenses tend to encode extremely subtle information, and, worse, their usages vary wildly from language to language -- it's rare to find any two languages that have tenses exactly equivalent to one another. (even in european languages that are closely related to english, different tenses are often used for the same situation than would be used in english.)

AMBER513

https://www.manhattanprep.com/gmat/forums/gains-in-stock-market-had-t10316.html?hilit=Before%20the%20introduction%20of%20soft%20landing%20mats
RON在帖子里讲了这个语法点，楼主如果还来CD的话可以看看

AMBER513

we have now realised是一个time marker,暗示前面一整句past tense,所以用past perfect,同意楼上的以句意为先

lisa001

遇到时态类的题目，建议题主优先考虑句意。
题目说臭氧保护人类免于辐射，在距离地球12英里的高空，那里曾经被认为是受不到人类活动影响的地方，但现在我们意识到工业废气会影响臭氧层。

说的是我们对臭氧的认识近一步加深的问题，我们现在认识到臭氧层是这样的，而原来是那样的，同一个主体先后两个动作，用现在完成时强调动作次序，没有毛病。

我不知道为啥时态会扯到句子结构上，如果一定要用分号分析的话，也可以这么理解：分号连接两个独立句子、且句意相关（曼哈顿出处）。两个独立句子的意思是结构完整，原文前后两句都有主谓宾，句子结构完整，没毛病。但题主没有关注语意，分号前后句句意一定相关。这里也是为什么我说是人们对臭氧认识近一步加深的意思，也是为什么能解释两个先后的认识的动作。

基础语法理解偏差的话，做不对时态题。

绿杨小庙

又找到一条这样的题目：
The success of the program to eradicate smallpox has stimulated experts to pursue what they had not previously considered possible--better control, if not eradication, of the other infections such as measles and yaws.

A what they had not previously considered possible--better control, if not eradication, of the other infections such as 分析该选项
B what they had not previously considered a possibility--better control, if not eradication, of such infections like 分析该选项
C something they had not previously considered possible-better control, if not eradication, of such infections as 分析该选项
D something not considered a previous possibility--better control and perhaps eradication, of other infections such as 分析该选项
E the possibility of what they had not previously considered--better control and possibly eradication of infections like 分析该选项
我的答案：A
正确答案：C

先复制一下不选A的原因：（by Ron）
Major problem is "THE other infections." THE is too definitive here, carrying the connotation of "every single one of the other infections."
* THE is also incompatible with "such as":
- Correct: I never read this book, but I read the other books on the shelf.
- Correct: I never read this book, but I read other books on the shelf, such as "Right Hand, Left Hand" and "The Rise and Fall of the Third Reich."
- Incorrect: I never read this book, but I read the other books on the shelf, such as "Right Hand, Left Hand" and "The Rise and Fall of the Third Reich."
- Minor problem is "what they had not..." vs. "something they had not..." The "what" construction is awfully strong, suggesting that this was THE ONE THING they hadn't thought possible.
* As an analogy, compare the meanings of "I want to do what I love for a living" and "I want to do something I love for a living." The first suggests that the speaker has one particular field in mind; the second doesn't.

尽管这道题并没有考过去完成时，但是通过正确选项可以发现，确实现在完成时可以作为过去的动作。因为has stimulated是过去的动作，而consider在stimulate之前，所以用过去完成时。
open to discuss.
希望各路英雄看到类似的例子能粘贴下，证明我的想法是对的还是错的。