摘要：He must be at home because his bike is here，此句基于自行车在家这一当下迹象，有力推测出他现在一定在家；

情态动词

表推测

肯定推测：must 表现在推测用 must do，过去推测用 must have done。

He must be at home because his bike is here，此句基于自行车在家这一当下迹象，有力推测出他现在一定在家；

He must have gone to bed as his room is dark，从房间黑暗这一现在状况合理推断出他过去一定已上床睡觉。

可能推测：may/might/could 表现在推测用 may/might/could do，过去推测用 may/might/could have done，可能性从 may 到 could 递减。

He may/might/could be in the library，说明他现在有可能在图书馆，但 may 表示可能性相对较大，might 和 could 可能性依次降低；

She may/might/could have forgotten about the meeting，意味着从现在看她过去有可能忘记会议，同样 may have forgotten 表示的可能性相对较高。

否定推测：can't 表现在推测用 can't do，过去推测用 can't have done。

He can't be in the classroom. I saw him go out，因看到他出去所以确定他现在不可能在教室；

He can't have stolen the money. He was with me，依据当时和他在一起断定他过去不可能偷钱。

其他用法

can/could：表能力时，

can 表现在能力

I can speak English

表明现在会说英语；

could 表过去能力

When I was young, I could run very fast 说明过去年轻时能跑得很快。表请求许可时，could 比 can 更委婉， Could you pass me the book 比 Can you pass me the book 更礼貌。

may/might：表许可，might 更委婉， May I use your pen 常见，而 Might I use your pen 则在更正式或对不太熟的人请求时用，显得更谦逊。

must：表必要性、义务，You must finish your homework 强调完成作业是必须的；其否定 mustn't 表禁止，如 You mustn't smoke here 明确禁止在此吸烟。

should/ought to：表建议、劝告， You should/ought to study harder 均是建议对方更努力学习，ought to 语气稍正式。表推测时，因有依据所以可能性较大，The film should/ought to be good as it has good reviews，基于电影评价好推测电影应该不错 。

来源：明日之星教育