表示說話人對某一動作或狀態的態度，表示/認為“可能”、“應當”、“必要”等含義，但情態動詞本身詞義不完全，不能單獨作謂語/謂語動詞，必須和動詞原形（不帶to的不定式）連用。情態動詞沒有人稱和數的變化。朗讀時，情態動詞的肯定式一般不重讀。

情態動詞的否定式一般是在它們的後面加否定詞not構成，如could的否定式是could not, 簡寫時一般在詞尾加-n’t, 如couldn’t; 但shall not的否定式應簡寫為shan’t(不常用簡寫形式)。

will, would

will表示“意志”、“意願”，可用於多種人稱。

I will (I’ll) tell you all about it.

We will (We’ll) help him if he asks us to.

He won’t go with us.

will在疑問句中用於第二人稱時，表示詢問對方的意願或向對方提出請求。 would是will的過去式，表示過去時間的“意志”、“意願”，可用於多種人稱。表示說話人的意願或向對方提出請求，語氣比will婉轉，指現在時間。

I’m going to the library. Will you go with me?

Will you give him a message when you see him?

They said that they would help us.

I promised that I would do my best.

Would you like to join the football club?

I’d like to take a look at that shirt over there.

shall, should

should常用來表示勸告、建議，認為某人“應該”、“應當”做某事， 或“有義務和責任”做某事。

-I have a headache.

-You should go to bed.

You should eat a lot of healthy food.

You shouldn’t watch TV every day.

在疑問句中，shall用來徵詢對方意見或請求指示，用於第一、三人稱。should 作為情態動詞，表示“勸告”、“建議”，常譯作“應該”。

Where shall I wait for you?

Shall we start the meeting now?

You should keep your promise.

Young people should respect old people.

can, could :

表示“能力；許可；可能性”等。could可以代替can表示語氣較為婉轉。在口語中can可以代替may表示許可，而may比較正式。

I can speak English and a little French. (能力)

Can we eat in school? (許可)

Could I watch TV? (許可)

Could you take out the trash? (可能性*)

Some of us can use the computer now, but we couldn’t ten years ago. (能力)

Can/Could I borrow your bike for a moment? (許可)

He said I could use his computer. (許可)

I thought the story could not be true. (可能)

may, might

表示許可或徵詢對方許可，有“可以”的意思。用作此意時，它的否定形式可以用may not, 但表示“不可以”、“禁止”、“阻止”等意思時常用must not (mustn’t) 代替代may not.

You may go now.

He said that I might use his telephone.

-May I go with my friends to the harvest festival?

-Yes, you may. / No, you mustn’t. / No, you may not. / No, you’d better not.

表示可能性，有“或許”、“可能”意思，may或might + 動詞原形都可表示可能性。用might則語氣更加不肯定。

He may / might be from Canada.

They may / might have a lot of work to do.

以上紅色助動詞只有過去式，沒有過去分詞 (其他的助動詞待確定)

dare

But as the moon gave far too much light, I didn’t dare open a window.

具有情態動詞和一般動詞兩種用法，通常與一般動詞一樣構成否定句和疑問句，後接帶to的不定式，最常用於否定句中。如：

I didn’t dare to go.

He won’t dare to break his promise.

dare 亦可作情態動詞，在英國英語中尤用於現在時否定式，後接不帶to的動詞不定式。

I daren’t tell her the news.

在口語中，該詞的各種形式(我認為指的是“一般動詞”和情態動詞)常與不帶to的不定式連用。如：

Don’t you dare tell her what I said! 你敢告訴他我說的話！

I didn’t dare look at him.

ought (否定式ought not to / oughtn’t to；疑問式為Ought I / you to …?)

無人稱和時態的變化，後接帶to的動詞不定式。 ought to 可表示“義務”、“要求”或“勸告”，常譯作“應該”、“應當”等（和should差不多，只是語氣稍重一些）；有時表示“非常可能”的意思

There’s something I ought to tell you before you leave.

He ought not to do that.

-Ought he to see the doctor?

-Yes, he ought to.

If she is completely well, she ought to be back at school today.

used to

had better

must（沒有過去式）

表示“必須”、“應該”。否定式must not / mustn’t 表示“不應該”、“不許可”、“不準”、“禁止”等。在回答帶有must的問句時，否定式常用need not / needn’t 或 don’t have to, 表示“不必”，而不用must not。 表示“一定”、“必定”等推測意義時，一般只用在肯定句中。

You must eat less meat.

You mustn’t speak like that.

-Must I be home before eight o’clock?

-Yes, you must.

-No, you needn’t. / No, you don’t have to.

The work must be finished as soon as possible.

You mustn’t smoke in public places.

There must be something wrong.

I don’t like this radio. I must buy a new one.

The radio is broken. I have to buy a new one.

have to（過去式：had to）

表示“必須”、“不得不”。在這個意義上與must很接近，但must表示的是說話人的主觀看法，而have to表示的卻是客觀需要。have to比must有更多的形式。

I must go now. (主觀看法)

I have to go now. (客觀需要)

You don’t have to worry about that.

You have to wear sports shoes for your gym class.

Every student will have to know how to use computers.

其他：

You have got to go now. (我造的句)

Have you got to go now?（書上句子）

will have to

need

The bell rings so you need to go to class. (這裡need應該是實義動詞)

“I need to pack up my things in the suitcase very quickly,” the girl said. (這裡need應該是實義動詞)