a great deal后面如果要跟名词,必须加介词of,即a great deal of,
注意：a great deal of只能修饰不可数名词.
PS:a great deal可以不加名词,这个时候它就被用作副词了.
例如：I've learnt a great deal from this book.
从这本书上我学到了很多.