情态动词和不带to的动词不定式一起构成谓语，但ought除外。情态动词有一定的词义，没有人称和数的变化。它表示说话人的语气或情态，如请求、命令、愿望、能力以及可能性等。英语的情态动词有：
can， could
may， might
must
ought to
shall， should
will， would
dare， dared
need
1. can， could
(1) 能，能够、会、可以
energy can be changed from one form into another. 能可以从一种形式变成另一种形式。
we can buy a car by instalments. 我们可以以分期付款买轿车。
(2)许可
you can ( = may) go home now. 你现在可以回家了。<>
(3)用于条件句中
if he comes， he can help us. 如果他来，他能帮助我们。
i could come earlier， if necessary. 如果有必要，我可早些来。
(4)表怀疑，难以置信
he couldn't (can't) be her father; he is too young.
(5) could + have + p. p. 婉转地责备、遗憾
he could have passed the exam. 他当时本该通过考试。
(6)be able to是can的同义结构，常用以表示cm所不能表示的未来或完成的概念。如：
①thus， we shall be able to determine the amount of alcohol burned. 这样，我们就可以确定燃烧掉的酒精量。
they have not been able to come. 他们没有能来。