不得不 + 做什么 = have to + do something 不得已 + 做什么 = you must do something you do not want to do,somebody else force you to do

”不得不“ Have to ”不得已“ Have no alternative but to

they are the same except that there is something special for 不得已. in fact 不得已 comes from the old chinese. for the expression, 不得已而为之, u cant say 不得不而为之 而: to 为: do 之 : something which we have known. example: -why do u do this? - 我不得已而为之 so, 我不得已而为之 = I have to. anyway, 不得已而为之 is an old chinese expression which we still use today. in old chinese, 已 means "control" 自已 means "to control myself" 不得已 means 不得自已 不得 means can not 不得自已 means can not control myself we omit the 自, then it becomes 不得已. so 不得已 + do something : can't control myself to do something (why can't I control myself? because something that I can't change or stop forces me to do something) 不得不 + do something : can't NOT to do something (double negative to express "have to do") for u, u just need to know two things: 1) normally, they are the same. 2) 不得已而为之

意思很相近，都有客观情况迫使人不得不或必须做某事的意思。 不得不（ have to do）必须做某事；但不同于“must”。 造句：时间太紧张了，我们不得不决定搭乘最近的航班去那里。 We have to catch the next flight to there due to the tight schedule. 不得已 （ act against one's will/ be forced to do/have no choice 等等），更强调心里很不情愿而又不得不去做的事的意思。 劝别人时可以这样说， 不到万不得已，你最好不要轻易这样做。 You ‘d better not to do it rashly, unless it’s absolutely necessary. In En,how would you say it? 其他造句： 我相信你是不得已才这样做的。（说明情非得已（unwillingness），而又别无选择） I believe that you were forced to do it . Or, I believe that you had no choice but to do so . 时间太紧了，不得已我们只能搭飞机去那了。（有点本不情愿坐飞机而迫于时间压力） We can only flight to there,due to the tight schedule. Hope this helps.

''不得不''：主观上自己认为 ''不得已''：有被迫的感觉，只能这么做，