2、表可能性can多用于疑问句、否定句，用于肯定句仅
指逻辑上或理论上的可能性 Mr Reed is in poor health. He can be ill at any time. 有生病的可能性。 Mr Reed looks pale. He may be ill.应该是病了
3、过去的可能：may / might / could / can +不定式的完成体 I don’t know where the letter is. I may / might have thrown it away.（might，could并非表过去，只是使语气更婉转，过去时间通过完成体形式体现。）（“can +不定式的完成体”只能用于否定句和疑问句中。）
4、“本可能发生但没有发生”：could / might have done You might have killed yourself. （不用may have killed）该句型表示与过去事实相反的假设，所以要用could，might等过去式的形式。
易错①* He ran after the bus, and could catch it.
②* You could pass the exam, but you weren’t careful enough.
①√ He ran after the bus, and was able to catch it.
②√ You could have passed the exam, but you weren’t careful enough.
其他表达法:probable, probably, likely, probability, likelihood 等
