a large number of people后面为什么用are不可数名词在什么情况下用are呢?除了a large number of还有什么类似的词?我记得有的是只能接可数名词的复数形式吧,

首先要搞清楚，是修饰可数名词或不可数名词，而不是单数或复数名词。可数名词都有复数形式，而不可数名词没有复数形式。
a large deal of、a plenty of和a good supply of 在多数情况下修饰不可数名词，如：a plenty of water, a geat deal of time，不过它们也可以修饰可数名词，如a plenty of men。
a quantity of在多数情况下修饰可数名词，如a quantity of boxes, 不过也可修饰不可数名词，如a quantity of milk。
quantities of与上述的一样
一、只能修饰可数名词的词语有:
many, many a(n), a good / great many, a (great / large) number of, scores of, dozens of等。例如:
Scores of people went there in the first few days after its opening.
开张后的头几天,很多人去了那儿。
I have been there dozens of times.
我已去过那儿很多次了。
There’re a number of students reading English in the classroom.
教室里有许多学生在读英语。
Many a student has(=many students have)visited the Great Wall.(谓语动词用单数)
很多学生都游览过长城。
In winter, a good many animals sleep under the snow.
冬天很多动物在雪下冬眠。
注意: many所修饰的复数名词前若有限定词,many 后面要接of, 表示“……中的很多”。 例如:
A great many(of the) graduates have found jobs.
毕业生中很多人已经找到了工作。
二、只能修饰不可数名词的词语有:
much, a great deal of, a great / large amount of等。例如:
Is there much water in the bucket?
桶里有很多水吗?
He always has a great amount of work to do.
他总是有很多工作要做。
三、既能修饰可数名词又能修饰不可数名词的词语有:
a lot of, lots of, plenty of (以上三个词语后谓语动词的数依of 后的名词的单复数而定),a great / large quantity of (其后谓语用单数),quantities of (其后谓语用复数)。例如:
There is still lots of snow in the garden.
花园里还有许多雪。
There is plenty of rain here.
这儿的雨水很多。
A great quantity of flowers was placed in the hall.
大厅里放了很多鲜花。
There are large quantities of food in the cupboard.
橱柜里有许多食物。
在所有这些表示“很多”的词语中many, much 是最常用的词,它们既可以用于肯定句,也可以用于疑问句和否定句。例如:
Are there many people in the street?
街上有很多人吗?
There isn’t much time left.
剩下的时间不多了。
其它的词语都用于肯定句,日常会话中常用lots of, a lot of 或plenty of; 正式文体中常用 a great many, a (large) number of, a great deal of, scores of 或 dozens of 等。但若肯定句中有too, so, as, very或how 等词修饰时,则必须使用 many, much。例如:
The number of the people who lost their homes reached as many as 250,000.
无家可归的人数多达250,000人。
There is too much work to do.
要做的工作太多了。..

people不是不可数名词,而是单复同型的特殊名词,一般用来指群体的人,单个的人一般用person,加之前面的 a large number of本身就是大量,许多的意思,所以后面必须用are.