各种考试常常考到先行词it充当主语 系动词 某些形容词 that从句，其中从句或用现在时虚拟语气（美国英语，即谓语动词直接用动词原形）或should 动词原形（英国英语），同学们往往出错，今天就一网打尽这个考点：
这类的形容词有：
appropriate（适当的;合适的;恰当的）[əˈprəʊprieɪt]
essential （重要的，不可少的，至关重要的，极其重要的，完全必要的）[ɪˈsenʃl]
important（重要的）
necessary（必须的）
imperative（非常必要的）
vital（至关重要的）
impossible（不可能的）
obligatory（必须的）
advisable（明智的）
desirable（好的，所希望的）
preferable（更好的）
compulsory（强制性的）
请看例句：
It is appropriate that Hispanic names dominate the list.
西班牙人的名字在这个名单中占主导地位是适当的。
It is appropriate that Irish names dominate the list
名单上大部分是爱尔兰名字，这没什么不合适的。
It is essential that you have some experience.
你必须得有些经验。
It is essential that we present a united front.
至关重要的是我们要表现得团结一致。
It was essential that he find the tomb first.
重要的是他先得找到这座骑士坟墓。《达芬奇密码》
It is essential that the meeting start at eight.
会议务必在8点钟开始。
It is essential that we should tell her the news.
我们有必要告诉她这个消息。
It is important that he attend every day.
他每天都要出席，这很重要。
It is important that he should attend every day.
他每天都要出席，这很重要。
It’s important that we (should) reply to her letter.
回复她的信是重要的。
It is important that we should speak politely.
我们说话要有礼貌，这是很重要的
It is important that we should speak politely.
我们说话要有礼貌，这是很重要的。
It is not necessary that everyone be a scientist.
没有必要使每一个人都成为科学家。
It is impossible that she (should) forget our meeting , she must have stayed away on purpose.
她不可能忘记我们的会议，她准是故意不来。
It's impossible that he finishes the work in three hours.
= It's impossible that he should finish the work in three hours.
= It's impossible for him to finish the work in three hours.
他不可能在三个小时之内完成这项工作。
说明这个词两种用法都可以。
It is necessary that I should return it this morning.
今天早上我就有必要还回去。
It is necessary, absolutely ,that I should return it this morning.
我绝对必须在今早把它还回去。
It is absolutely imperative that we finish by next week.
我们的当务之急是必须于下周完成。
It was imperative that he act as naturally as possible.
最重要的是他要做到尽可能自然。
It is vital that you keep accurate records when you are self-employed.
干个体的要准确记录账目，这十分重要。
It is vital that we act to stop the destruction of the rainforests.
至关紧要的是，我们应当采取行动制止破坏雨林。
It's vital we are as fresh as possible for those matches
尽可能保持旺盛的精力参加那几场比赛对我们来说至关重要。柯林斯高阶英汉双解学习词典》
说明这个词两种用法都可以。
It’s impossible that he (should) marry her.
他同她结婚是不可能的。
It is advisable that everyone should have a map.
最好每一个人都有一张地图。It is advisable that he should see a dentist regularly.
他定期去看牙科医生是明智的。
It is desirable that everything should be ready by five o’clock.
最好一切在5点前准备就绪。
It is desirable that there should be a cold season once a year.
每年有一个寒冷的季节，是很令人盼望的。
It is desirable that interest rates should be reduced.
利率下调是可取的。
It is desirable that interest rates be reduced.
利率下调是可取的。
