A 用于推测过去的行动：
He may/might have gone.相当于：
It is possible that he went/has gone./Perhaps he went/has gone.
当时他可能去了。
如果主句的动词是过去时，就必须用might：
He said/thought that she might have missed the plane.
他说/想她可能已经误了那班飞机。
如不确定性已然消失，必须用 might而不用 may：
He came home alone.You shouldnt have let him do that;he might have got lost.
他独自回家了。你本不应当让他那么做的，他可能会迷路的。(但他并未迷路。)
在下一句中也是这样：
You shouldnt have drunk the wine;it may/might have been drugged.
你本不应喝那酒的，里边可能放了药。
it may have been drugged通常表示还不能确定酒里是否放了迷药。
it might have been drugged也可具有与上述相同的含义，但也可能意指现在知道酒里没有迷药。
在叙述最终并未得到检验的事情时用 might而不用 may：
Perhaps we should have taken the other road.It might have been quick- er.
也许我们本应当走另一条路。那样可能更快些。
Its a good thing you didnt lend him the money.You might never have got it back.
你没借给他钱倒是好事。如果你借给了他，就可能永远也要不回来了。
上述这类例句同条件句的第三种类型非常相似：
If we had taken the other road we might have arrived earlier.
如果我们当时走了另一条路，就可能到得早一些。
B may/might可用于条件句中以代替will/would，表示一种可能的而不是肯定无疑的结果：
If he sees you he will stop.
如果他看见你，他就会停下来。(确定的)
If he sees you he may stop.
如果他看见你，他可能停下来。(可能的)
同样， may/might也可用于下列句子当中：
If you poured hot water into it，it might crack.
你如果往它里面倒进热水的话，它就可能裂开。
If you had left it there someone might have stolen it.
要是你当时把那东西落在那里，就可能有人把它偷走。