a great many 是一个固定的搭配结构,意思是很多的,非常多的,只能用来修饰可数名词 .a great在这 是修饰加强 many.
还有一些修饰可数名词的词或词组如：a good many,a number of , many , few
修饰不可数名词的有： a great dealof,a large amount of ,much,little
即修饰可数又修饰不可数的有：plenty of,some,any, a lot of , lotsof,a mass of,masses of ,a large quantity of, quantities of
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