情态助动词，顾名思义，是帮助实义动词表达说话人的情感和态度的动词。
1. need/dare
1)need和dare既可以作实义动词(所以后能加实义动词的非谓语动词形式to do等)又可作情态助动词(所以可直接加实义动词原形do）
2)实义动词变否定时前加基本助动词do,情态助动词变否定时直接加not.
不需要做：doesn't need to do=needn't do
不敢做：doesn't dare to do=dare not do
2.must
1)必须（表义务和必要≈have to）。
---- Must we hand in our exercise books today?
----Yes, you must.
----No, you don’t have to =you needn’t.（不必）
注意：不能用musn't（绝不）回答，musn't表强烈禁止。
2)肯定（表示可能性大的推测,只用于肯定的陈述句）
You're Tom's good friend, so you must know what he likes best.
3)偏要，非得（表不悦不耐烦）
Must you make so much noise?
3.will/would
1)将会（表预测或临时决定≠be going to）
I think it wont rain tomorrow.
注意：与前文时态难点1：一般将来时只能用will do？中be going to的辨析。
2)愿意（表意愿或执意≈be willing to）
If he will pay by check, we have to accept it.
注意：will在此是情态助动词，表执意，不是表将来时，否则就不符合主将从现的规则了。
3)过去常常做…(≈used to do)
He would go swimming in this river when he was young.
注意：would只表动作，used to do可表动作或状态。
She used to be very shy when she was young.
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