一、情态动词的特征
1．情态动词表示说话人对某一动作或状态的态度，有一定的词义，但不能单独作谓语，必须和动词原形，即不带to的不定式连用，才能构成谓语。
2．情态动词没有人称和数的变化(have to例外，主语是第三人称单数时，要用has to)。
二、情态动词的基本用法
1．be able to/can/could
[观察例句]
①This time I failed，but I'll be able to pass the exam next time.
②Mary can speak three foreign languages.
③He was able to leave Germany for America during the Second World War.
④Tom could sing English songs at the age of 6.
⑤What can they be doing there?
⑥He can't be in the classroom，for the light has been turned off.
⑦It can be quite cold at night here.
⑧—Could I use your bike?
—Of course you can.
