should是不是动词原形（动词原形的虚拟语气几个值得注意的知识点）

1、在for fear that，in case，lest引导的从句中，若用虚拟语气时，从句谓语为：（should） do。并且should能省略（for fear that，lest），但是in case不能省略

She examined the door again for fear that a thief (should) come in.

她又把门检查了一遍，以防盗贼的进入。

He started out earlier lest he (should) be late.

他早早地就出发了以防迟到。

2、在so that，in order that所引导的目的状语从句中，从句中的谓语为：can / could / may / might / will / would /should do

He goes closer to the speaker so that he can hear him clearer.

他走近说话的人以便能听得更清楚。

He read the letter carefully in order that he should not miss a word.

他把信读得很仔细以便不漏掉一个字。

一想要(desire) 一宁愿(prefer) 一坚持(insist)二命令(order ， command) 三建议(advise ， suggest ， propose/recommend) 四要求(demand ， require ， request ， ask)中，无论主句谓语动词为何种时态，从句的谓语动词都用："should do"。should可以省略。

eg：He suggested that we (should) take the teacher's advice.

He insisted that we (should) take the teacher's advice.

He demand that we (should) take the teacher's advice.

He ordered that we (should) take the teacher's advice.

insist意为“坚持某种动作”才用虚拟语气；意为“坚持某种观点，某个事实”则不用虚拟语气。

语法引导条件状语从句最常用的连词是if,由if引导的条件状语从句表示在某种条件下某事很可能发生。如：

1） If you ask him,he will help you.如果你请他帮忙，他会帮你的。

2） If you fail in the exam,you will let him down.如果你考试不及格，你会让他失望的

if引导的条件状语从句既可以将从句放前面也可以将从句放后面

（如果天下雨，我们就不玩了）If it rains,we will stop playing.‘

转为We will stop playing if it rains.

另外，if从句还表示不可实现的条件或根本不可能存在的条件，也就是一种虚拟的条件或假设，从句多用一般过去时或过去完成时。

如：

If I were you,I would invite him to the party.如果我是你，我会邀请他参加聚会。

I would have arrived much earlier if I had not been caught in the traffic.如果没有堵车，我会到的早一点儿。

unless = if...not.

e.g. Let's go out for a walk unless you are too tired.

If you are not too tired,let's go out for a walk.

if 引导的条件句有真实条件句和非真实条件句两种：

真实条件句

如：If it rains tomorrow,we won’t go on a park

非真实条件句是虚拟语气的一种，表示与事实相反，如：

If I were you,I would go with him.

so/as long as只要

由as (so) long as,in case引导。

So long as you’re happy,it doesn’t matter what you do.

只要你高兴，你做什么都没有关系。

You may borrow my book as long as you keep it clean.

只要你保持书的清洁，你就可以把我的书借去。

Take your umbrella in case it rains.

带着你的伞吧，以防下雨。