demand，require和request都可以表示“请求，要求”,但是用法不同：
1.demand表示“要求，需要”时，语气较强，有强烈要求的含义，表示行为者认为自己有权这样做。（尤指困难、使人劳累、令人烦恼等的）要求;还可以表示需求，如客户需求。
主语是人时，表示坚决要求，坚持要做某事；主语是物时，指迫切需要。
demand后面接of sb. to do sth.
Our teacher demanded of us to finish homework.
我们的老师要求我们完成作业。
She demanded an immediate explanation.
她强烈要求立即作出解释。
Mr Byers last night demanded an immediate explanation from the Education Secretary
昨晚拜尔斯先生要求教育大臣立刻作出解释。
She demands that she be told everything. 她要求把一切都告诉她。
He demanded an explanation. 他要求解释。
I demand to see the manager.我要求见经理。
She demanded that we （should）let her in.
她要求我们让她进来。
She demanded that he （should）return the books he borrowed from her.她要求他把从她那里借走的书还回来。
Russia demanded that Uinta （should）send a delegation to the peace talks
俄罗斯要求安盟派代表团参加和谈。
The UN has demanded that all troops （should）be withdrawn.
联合国已要求撤出所有部队。
2. require表示依据权利、条例、规章而“要求”，带有命令语气，常用于被动语态，语气比demand缓和。这里的“要求”是指一定要有的东西和条件。侧重于少了这个不行，不被接受。尤其表示按照法规、权利提出的要求或命令：
We shall fulfill our obligations as required by law.
我们应当根据法律的要求履行自己的义务。
The wearing of seat belts is required by law.
法律规定必须系安全带。
The police required the traveler to unlock his suitcase.
警察要求这位旅客将手提箱打开。
All candidates will be required to take a short test.
所有候选者都要参加一个小测验。
The rules also require employers to provide safety training.
这些规定还要求雇主提供安全培训。
We require details of your name and address.
我们需要你的姓名和地址的详细信息。
Babies require a lot of care and attention.
婴儿需要悉心照顾。
The patient requires an operation.
这个病人需要手术。
This condition requires urgent treatment.
这种情况得紧急处理。
You are required by law to send your children to school. 法律要求你送孩子上学。
True marriage requires us to show trust and loyalty.
真正的婚姻有赖于我们表现出信任与忠诚。
The rules also require employers to provide safety training.这些规定也要求雇主们提供安全训练。
He requires you to reply immediately. 他要求你立即回答。
Legislation will require UK petrol companies to meet an EU specification for petrol.
法规将要求英国的汽油公司必须达到欧盟对汽油的规格要求。
Girls do better on open-ended tasks that require them to think for themselves.
女孩子更擅长做需要独立思考的开放式任务。
The situation required that he （should）be present.
这种情形需要他在场。
His health requires that he (should) stay at home. 他的健康状况要求他待在家里。
Lentils do not require soaking before cooking.
小扁豆在烹饪前不必浸泡。
提示
ask也可表示“要求”，指的是要求某人做某事或向某人要求什么东西。
She asked me for help. 她请求我帮助。
He will ask his father for more money. 他将请求父亲多给一些钱。
3. request表示客气有礼貌的正式请求、要求。（礼貌、正式的）要求，请求；一般是带有“请求”的含义。指通过正式手续提出的要求，口气和缓，态度礼貌。郑重地要求和希望时，用request。
Visitors are requested not to smoke in the hall。在大厅要求宾客们禁烟。
You are requested not to talk here. 这里请勿交谈。
They requested a loan. 他们请求贷款。
May I request a favor of you?
请您帮一下忙好吗?
My brother requested me to help him.
我的弟弟请求我帮他。
Kate make a request for more vacation time. Kate
要求更多的休假时间。
She requested permission to film at the White House.
她申请准予在白宫拍摄。
He requests to use the machine. 他请求使用那台机器。
He requests to use the machine. 他请求使用那台机器。
Mr Dennis said he had requested access to a telephone.
丹尼斯先生说他已经请求使用电话。
You can request a free copy of the leaflet.
你可以索要一份免费的宣传单。
A United Nations spokeswoman in New York said the request would be considered.(名词)
联合国的一名女发言人在纽约说，该要求会得到考虑。
That country made an urgent request for international aid after the tsunami happened.
海啸发生后，该国紧急请求提供国际援助。(名词)
The opposition leader repeated his request that they release the election results as soon as possible.
反对党领导人再次要求他们尽快公布选举结果。(名词)
Request、require、demand后面跟宾语从句时，从句后谓语动词都要用虚拟语气，即should加动词原形。（名词性从句也是如此）
The teacher requested that each of us (should) go there at once.
老师要求我们马上到那儿去
这里的request可以替换成require 和demand。
