① allow意為「准許，允許」，指允許或聽任別人做某事，常表示答應，側重默許;allow也可以表示客氣的請求，結構為：
allow sth./doing sth.
allow sb.+sth. (雙賓語)
allow sb./oneself to be+形容詞(承認……)
allow sb. to do sth.
be allowed to do sth.
We can't allow such a thing. 我們不能聽任這種事情發生。
They didn't allow him to stay there. 他們不允許他待在那裡。
You must allow yourself to be wrong. 你必須承認你錯了。
We don't allow smoking in the room. 不允許在房間裡吸菸。
She was allowed to try again. 她被允許再試一次。
Please allow me to say a few words. 請允許我說幾句話。(客氣請求)
比較
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意為「允許，許可」，語氣比allow更正式，更強，表示的是積極允許，側重「允許」，結構為：
permit sth./doing sth.
permit sb.+sth. (雙賓語)
permit sb. to do sth.
if time/weather/health permits
We should not permit the waste of money. 我們不允許浪費錢財。
We don't permit smoking in the office. 我們不允許在辦公室里抽菸。
Mother doesn't permit me to stay up late. 母親不許我熬夜。
I'll call on you if time permits. 如果時間允許的話，我將去看你。
提示
①permit和allow有時可通用，後面都可以跟動名詞作賓語，都可以表示客氣的請求。比較：
他們不許在這個湖裡游泳。
They don't permit swimming in the lake.(正式說法)
They don't allow swimming in the lake.(一般說法)
Please permit me to explain. 請允許我作解釋。
Please allow me to explain.
②permit和allow後不可跟不定式，也不可跟從句。比較：
They don't permit/allow to sit here.
They don't permit/allow that we sit here.
They don't permit/allow sitting here.
他們不允許坐在這裡。
③permit和allow的賓語補足語可以是帶to的不定式，但不能是現在分詞、動名詞或不帶to的不定式。比較：
她不允許他們拍照。
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非人的東西作主語時可用permit，也可用allow。
The plan allows 20 minutes for dinner. 根據計劃安排，允許20分鐘時間吃飯。
【注意】表示下級對上級、幼輩對長輩的尊敬的請求，主語為you時，應用permit;主語為I時，應用May I be allowed…?
Will you permit me to say a few words?請允許我說幾句話好嗎?
May I be allowed to say a few words?
③ let意為「讓，允許」，指不加反對或阻止，也可用於命令、建議等，可指積極的允許，結構為：
let sb./sth. do sth. (不帶to)
let sb./sth. in讓……進來
let sb./sth. into sth. 讓……進入
let sb./sth. out放出，泄露，發出
let sth.(boat，room，house)出租
Please let me know what has happened. 請告訴我發生了什麼事。
Don't let the dog in. 不要把狗放進來。
Who let you into the office?誰讓你進入辦公室的?
Mother won't let her out on Sundays. 母親不讓她星期天外出。
She has a house to let. 她有一所房子出租。
提示
①let通常不用於被動語態;如要用被動語態，常用be allowed代替be let。
Mother didn't let him go swimming. 母親不許他去游泳。
He was not allowed to go swimming.
但let表示「出租」時，可用於被動語態。
The house was let last week. 這所房子上周被出租了。
②表示徵求他人的意見時，let's包括被徵求意見的人和說話人本人在內，意為「讓我們」，反意問句用shall we;而let us僅指說話人本人在內，被徵求意見的人you為句子主語，被省略了，反意問句用will you。比較：
Let's leave，shall we?讓我們離開吧，好嗎?
Let us leave，will you?你讓我們離開吧，好嗎?
④ promise意為「答應，允諾」，指答應某人的要求，提出某種積極的許諾，執行這種承諾的人就是句子的主語，結構為：
promise sth.
promise sb.+sth. (雙賓語)
promise to do sth.
promise sb. to do sth.
promise+that從句
keep the promise (to do sth.，that從句)
I can't promise anything. 我不能作任何承諾。
She promised him a birthday present. 她答應給他生日禮物。
He promised me not to go there again. 他答應我不再去那裡了。
She promised that she would lend me some money. 她答應借給我一些錢。
She broke her promise to lend him the car. 她未能履行把車借給他的承諾。