allow，permit，let和promise
① allow意为“准许，允许”，指允许或听任别人做某事，常表示答应，侧重默许；allow也可以表示客气的请求，结构为：
allow sth./doing sth.
allow sb.+sth. （双宾语）
allow sb./oneself to be+形容词（承认……）
allow sb. to do sth.
be allowed to do sth.
We can't allow such a thing. 我们不能听任这种事情发生。
They didn't allow him to stay there. 他们不允许他待在那里。
You must allow yourself to be wrong. 你必须承认你错了。
We don't allow smoking in the room. 不允许在房间里吸烟。
She was allowed to try again. 她被允许再试一次。
Please allow me to say a few words. 请允许我说几句话。（客气请求）
比较
We don't allow smoking here.
We don't allow to smoke here.
Mother did not allow her to go out.
Mother did not allow her going out.
② permit意为“允许，许可”，语气比allow更正式，更强，表示的是积极允许，侧重“允许”，结构为：
permit sth./doing sth.
permit sb.+sth. （双宾语）
permit sb. to do sth.
if time/weather/health permits
We should not permit the waste of money. 我们不允许浪费钱财。
We don't permit smoking in the office. 我们不允许在办公室里抽烟。
Mother doesn't permit me to stay up late. 母亲不许我熬夜。
I'll call on you if time permits. 如果时间允许的话，我将去看你。
提示
①permit和allow有时可通用，后面都可以跟动名词作宾语，都可以表示客气的请求。比较：
他们不许在这个湖里游泳。
They don't permit swimming in the lake.（正式说法）
They don't allow swimming in the lake.（一般说法）
Please permit me to explain. 请允许我作解释。
Please allow me to explain.
②permit和allow后不可跟不定式，也不可跟从句。比较：
They don't permit/allow to sit here.
They don't permit/allow that we sit here.
They don't permit/allow sitting here. 他们不允许坐在这里。
③permit和allow的宾语补足语可以是带to的不定式，但不能是现在分词、动名词或不带to的不定式。比较：
她不允许他们拍照。
She didn't permit/allow them taking photos.
She didn't permit/allow them take photos.
She didn't permit/allow them to take photos.
④非人的东西作主语时可用permit，也可用allow。
The plan allows 20 minutes for dinner. 根据计划安排，允许20分钟时间吃饭。
【注意】表示下级对上级、幼辈对长辈的尊敬的请求，主语为you时，应用permit；主语为I时，应用May I be allowed…？
Will you permit me to say a few words？请允许我说几句话好吗？
May I be allowed to say a few words？
③ let意为“让，允许”，指不加反对或阻止，也可用于命令、建议等，可指积极的允许，结构为：
let sb./sth. do sth. （不带to）
let sb./sth. in让……进来
let sb./sth. into sth. 让……进入
let sb./sth. out放出，泄露，发出
let sth.（boat，room，house）出租
Please let me know what has happened. 请告诉我发生了什么事。
Don't let the dog in. 不要把狗放进来。
Who let you into the office？谁让你进入办公室的？
Mother won't let her out on Sundays. 母亲不让她星期天外出。
She has a house to let. 她有一所房子出租。
提示
①let通常不用于被动语态；如要用被动语态，常用be allowed代替be let。
Mother didn't let him go swimming. 母亲不许他去游泳。
He was not allowed to go swimming.
但let表示“出租”时，可用于被动语态。
The house was let last week. 这所房子上周被出租了。
②表示征求他人的意见时，let's包括被征求意见的人和说话人本人在内，意为“让我们”，反意问句用shall we；而let us仅指说话人本人在内，被征求意见的人you为句子主语，被省略了，反意问句用will you。比较：
Let's leave，shall we？让我们离开吧，好吗？
Let us leave，will you？你让我们离开吧，好吗？
④ promise意为“答应，允诺”，指答应某人的要求，提出某种积极的许诺，执行这种承诺的人就是句子的主语，结构为：
promise sth.
promise sb.+sth. （双宾语）
promise to do sth.
promise sb. to do sth.
promise+that从句
keep the promise （to do sth.，that从句）
I can't promise anything. 我不能作任何承诺。
She promised him a birthday present. 她答应给他生日礼物。
He promised me not to go there again. 他答应我不再去那里了。
She promised that she would lend me some money. 她答应借给我一些钱。
She broke her promise to lend him the car. 她未能履行把车借给他的承诺。