虚拟语气：表示和已经发生的事情是相反的，或者未来发生的可能性很小。在 GMAT 句子改错中，常见的虚拟语气主要是有 if 引导的，还有名词性从句的虚拟。
If 条件状语从句的虚拟形式有 3 种，分别是和过去虚拟、现在虚拟和将来虚拟。要注意的是主从句动词形式的变化。
过去： If I had done, I would/could have done...
现在： If I were a bird, I would/could do...
将来： If it were to do, I would/could do...
和过去虚拟，表示和过去事实相反，比如
If I had known you earlier, I would have passed GMAT.
和现在虚拟，表示现在不太可能发生的事。如
If I were a bird, I would fly in the sky freely.
和将来虚拟，表示未来发生可能性很小。如
If the asteroid were to strike the earth, I would die.
PS: if 条件句有倒装结构，若从句中有 had/were/should 出现时，把这些词提前，省略 if 形成倒装结构。
If you had closed the door, you would not have lost your money.
-- Had you closed the door, you would not have lost you money.
GMAT 真题：
Scientists have identified an asteroid, 2000 BF19, that is about half a mile wide and, if it strikes Earth, it can do tremendous damage to part of the planet but probably not cause planetwide destruction.
A and, if it strikes Earth, it can do tremendous damage to part of the planet but
B and, if it would strike Earth, part of the planet could experience a tremendous amount of damage but it would
C and that, if it were to strike Earth, could do tremendous damage to part of the planet but would
D and that, if Earth is struck by it, can do part of the planet tremendous damage, but it would
E and that, if it strikes Earth, it could experience a tremendous amount of damage but
雷哥GMAT解析：
if 从句是和将来虚拟，因此从句是 were to do,主句是 could do.答案 C
名词性从句的虚拟也有 3 种，宾语从句、同位语从句和主语从句的虚拟。
1.宾语从句。
Advise、suggest、require、request、propose、recommend、command 等表示建议命令要求的动词，后面接宾语从句时，从句中的谓语要用动词原形。
Sb. require that....do...
Eg: He suggests that we hold a meeting next week.
雷哥GMAT温馨提示：在 GMAT 句子改错中，宾语从句的谓语动词不能添加 should.
GMAT 真题：
The commission proposed that funding for the park's development, which could be open to the public early next year, is obtained through a local bond issue.
A. that funding for the park's development, which could be open to the public earlynext year, is
B. that funding for development of the park, which could be open to the public earlynext year, be
C. funding for the development of the park, perhaps open to the public early nextyear, to be
D. funds for the park's development, perhaps open to the public early next year, be
E. development funding for the park, which could be open to the public early nextyear, is to be
雷哥GMAT解析：
propose 引导宾语从句，从句中谓语用动词原形，is 的原形是 be,答案 B.
2.同位语从句
表示建议、命令、要求等动词的名词形式出现时，同位语从句也要用虚拟语气。从句中谓语用动词原形。
GMAT 真题：
The budget for education reflects the administration's demand that the money is controlled by local school districts, but it can only be spent on teachers, not on books, computers, or other materials or activities.
A. the money is controlled by local school districts, but it can only be spent
B. the money be controlled by local school districts, but it allows them to spend themoney only
C. the money is to be controlled by local school districts, but allowing it only to bespent
D. local school districts are in control of the money, but it allows them to spend themoney only
E. local school districts are to be in control of the money, but it can only spend it
雷哥GMAT解析：demand 后面的同位语从句要用虚拟语气，is 的动词原形是 be,答案 B.
3.主语从句
表示个人对事件的反应，比如 strange/necessary/important 等。主语从句中谓语要用动词原形。
Eg: It is necessary that we make everything ready ahead of time.
GMAT 真题
Since chromosome damage may be used by viral infections，medical X-rays，and
exposure to sunlight， it is important that the chromosomes of a population to be tested for chemically induced damage be compared with those of a control population．
(A)to be tested for chemically induced damage be compared with
(B)being tested for damage induced chemically are compared with
(C)being tested for chemically induced damage should be compared to
(D)being tested for chemically induced damage are to be compared to
(E)that is to be tested for chemically induced damage are to be comparable with L雷哥GMAT解析：it is important that....主语从句中虚拟，谓语用动词原形 be,并且 should 必须省略。答案 A.
