首先要看出Were it not for the fusion-powered heat and radiation that rush from its core = If it were not for the fusion-powered heat and radiation that rush from its core
然后就会知道这是个虚拟语气/条件状语从句
A/B的Instead of碰上 if it were是赘余的
C/D成分没有差异，对比时态 C的正在进行时没有题目中没有依据
E 虽然毕老师的解释中说是不定式短语的问题，但是我一般看不出来这种问题。我认为如E所写，句中的it就无所指代了（GMAT一般不会去指代a star's collapse里面的star，我没见过这样指代对的）

虚拟语气的句子一开头就要是主语！

【Were it not 虚拟语气】for the fusion-powered heat and radiation that rush from its* core, 【a star 主语 】would collapse under its own weight
要不是核聚变产生的热量和辐射从它的核心涌出，【一颗恒星】会在自身重量下坍塌

A. 前面it指代后面主句的主语，成了指代its own weight，错误；这样的虚拟语气句子， 主句必须一开头就 是主语，而不能是连词、副词，因此instead错误； its指代不清
B. Instead错误同A；本句子描述的是当下的一个假设情形下的一个假设结果，要用现在时态，用would have collapsed表示了“与过去相反(也就是表示过去没崩溃)”不对。
C. Would have to be collapsing是很混乱的时态； would have to是不很肯定的语气，其表达的是have to do 的内容只是一个假设，并不是真的“必须”，而have to表达的意思是，要做的事情非常重要，“必须” 去做
D. correct
E. it指代了its own weight，错误； its指代不清； would have caused错误同B。

首先要看出Were it not for the fusion-powered heat and radiation that rush from its core = If it 【were】 not for the fusion-powered heat and radiation that rush from its core
然后就会知道这是个【虚拟语气/条件状语从句】！

A/B选项：instead a star would have collapsed under its own weight语法有误，虚拟语气句子，主句必须一开头就是主语，而不能是连词、副词，因此instead错误。
D选项：a star 【would】 collapse under its own weight正确

Were it not for the fusion-powered heat and radiation that rush from its core = If it were not for the fusion-powered heat and radiation that rush from its core
主句不能有instead，AB错误
C. would have to be 错误
E. would have done是“本可以...但没有“，是过去式，错误

1.句子结构，这是个强调句啊！要不是因为....，星星就可以...
后面一定要是个完整的句子，而且主句必须一开头就是主语，AB错
2.动词时态，CE的have肯定是错的。
3.句意推理，have caused a star's collapse，E的这个句意错了，是哪个星星的？不是他本身？E错

如果没有来自其核心的聚变热和辐射，一颗恒星会在自身的重量下坍缩。

A选项：语法有误，虚拟语气句子，主句必须一开头就是主语，而不能是连词、副词，因此instead错误。

C选项：would have to be collapsing是很混乱的时态；would have to是不很肯定的语气，其表达的是have to do的内容只是一个假设，并不是真的“必须”，而have to表达的意思是，要做的事情非常重要，“必须”去做，前后矛盾。

e还错在时态，参考曼哈顿关于if的5种时态用法
if 虚拟，then would do
if had done，then would have done

虚拟语气句子，主句必须一开头就是主语

e的it指代也很模糊吧

if it's not表示如果不是，则是；instead是副词，表示相反地，却。If it's not A, instead it's B构句法上一主一从是没有问题的，但语意上是冗余的。
要用instead的话，改成：It's not A, but instead it's B

Were it not = if it were not
倒装+虚拟语气

m

If it were not for the fusion-powered heat and radiation, then a star would collapse under its own weight.要不是因为热和辐射，星星会崩塌在自己的质量里。
it这里做形式主语，起强调作用。也可以是If not for the fusion-powered heat and radiation，……
A，if接then，instead错误
B，C，D，would have错误

if/then时态
主句从现（通常）
if did/were to，then would
if had done, then would have done
if should, then would

AB选项：if it were not和instead之间的重复
CE选项：时态有误

But for N, S + should/would/cold/might + V / (have Vpp.) 若不是N, S应该….
= If it were not / (had not been) for N
= Were it not for N / (Had it not been for N)
= Without N（介词只有without可表虚拟语气）

1. if … then..可以代表 如果..那么…
2. CD：成分一致的情况下才看时态，明显现在进行时不适用于此
重点：严谨逻辑的辨析
CD说的是，一个星星在这种重量下坍塌；而E说的是，重量造成了星星的坍塌；
此处有两点问题：1. cause应该是一件事情而不是一个东西；2. 重量不能构成造成星星坍塌的主要施动者，应该是在某种重量状况下坍塌。
在GMAT考试中，逻辑严谨的辨析非常重要，如果A能推出B,B能推出C，GMAT绝不接受A能推出C。

E的问题只是在于不定式和名词的区别吗？

Were it not for意思是：如果不是的话，和instead意思重复了。所以A,B错误。同时需要注意的是前半句用到了代词it和its，所以后半句需要明确指出代词指代对象，通过代词its core这里就可以判断出its指代的是star。最后，would do 和would have done的区别在于，would do是指将来，would have done是指过去应该做什么，完全不同。

But for N, S + should/would/cold/might + V / (have Vpp.) 若不是N, S应该….
= If it were not / (had not been) for N
= Were it not for N / (Had it not been for N)
= Without N（介词只有without可表虚拟语气）
虚拟语气的时态：
时间 从句谓语形式 主句谓语形式
将来 动词过去式(be用were)
should + 动词原形
were to + 动词原形 would / should / might / could + 动词原形
现在 动词过去式(be 用 were) would / should / might / could + 动词原形
过去 had +动词过去分词 would / should / might / could have + 动词过去分词