6．　 在有些句型中如：It is important (necessary、natural、strange等) + that从句中的谓语动词用should + 动词原形，should可以省略。例如：

It is necessary that the doctor (should) be sent there at once.

5．　 在有些动词如：suggest、insist、order等动词后，从句谓语动词用“should + 动词原形”，should可以省略。例如：

He suggested that we (should) discuss the problem right now.

4．　 动词wish后的宾语从句中，谓语动词可用过去式、过去完成式或would、might加原形。

例如：I wish I were as strong as you.

He wished I would stay with us.

She wished I had attended the meeting yesterday.

3．　 表示与将来事实相反的谓语形式。

例如：If they came tomorrow, we would told them about it. (In fact, they won’t come tomorrow。)

2．　 表示与过去事实相反的谓语形式。

例如：If I had left a little earlier, I would have caught the early bus. (In fact, I didn’t catch the bus.)

虚拟语气表示说话人的意愿、假设、猜测或建议。虚拟语气不表示客观存在的事实，谓语动词用特殊的形式表示虚拟语气。

1．　 表示与现在事实相反时的谓语形式。

例如：If Tom were hungry, he would eat the bread. (In fact, Tom isn’t hungry.)

4. must与have to比较

must侧重于个人意志和主观上的必要，have to侧重于客观上的必要。如果用于过去时态或将来时态时，则要用have to的相应时态来代替must。但must可用于间接引语中表示过去的必要或义务。

My brother was very ill，so I had to call for the doctor in the middle of the night.

我弟弟病得很厉害，我只得半夜里把医生请来。(一般过去时)

I haven't got any money with me，so I'll have to borrow some from my friend.

我身上没有钱，只好向朋友借点了。(一般将来时)

He said that they must work hard.他说他们必须努力工作。

3. have to的疑问句

句型：Do(Does, Did, Will)+主语+have to +动词原形？

Do you have to study maths now？

你现在必须学数字吗？

Yes，I do.是的，必须学。

No，I don't(have to)。不，不必学。

Did he have to ask the question？

他非要问那个问题吗？

Yes，he did.是的。

No，he didn't(have to).不。

2. have to的否定句

句型：主语+don't(doesn't, didn’t, will not)have to+动词原形

You don't have to walk so fast.

你不必走那么快。

He will not have to buy a new coat next year.

明年，他没必要买新外衣了。

1. have(has)to +动词原形

have(has)to后面要用动词原形。当主语为第三人称单数时用has to，当句子是过去时用had to。

We have to leave now.

我们不得不现在就离开。

He has to work on Sunday.

他不得不在周日工作。

I had to do my homework last Sunday.

上周日，我不得不做作业。