当it作形式主语，后面用形容词作表语，即在it is + adj.这样的句型中，当描述主语的表语形容词是advisable，critical，crucial，desirable，essential，imperative，important，incredible，inevitable，necessary，recommend，strange，urgent等形容词时，主语从句的谓语动词多用虚拟语气，如果是现在时，谓语动词用should/would +动词原形（should/would经常省略）；如果是过去时，则用should/would + have done的形式。
example 121.（2000年Text 1）
真题回放：It was inevitable that this primacy should have narrowed as other countries grew richer. Just as inevitably，the retreat from predominance proved painful.
真题回放：“Science never has all the answers. But science does provide us with the best available guide to the future，and it is critical that our nation and the world base important policies on the best judgments that science can provide concerning the future consequences of present actions.”
破解真题：我们重点看一下……and it is critical that……of present actions这一部分内容，这是一个含有主语从句的复合句。It是形式主语，真正主语是that our nation……of present actions这个从句；该主语从句中，谓语动词base是虚拟式，前边省略了should表示与现在事实相反。
真题回放：Theories concerning on the individual suggest that children engage in criminal behavior because they were not sufficiently penalized for previous misdeeds or that they have learned criminal behavior through interactions with others.
真题回放：Such behaviour is regarded as“all too human”，with the underlying assumption that other animals would not be capable of this finely developed sense of grievance.
真题回放：Another decision is whether the school should be one of the vast majority financed by the State or one of the very small but influential minority of private schools，though this choice is，of course，only available to the small number of those who can pay.
