1.a lot有副词性,可用作状语;a lot也有名词性,可用作主语、宾语、表语等。如:
Im feeling a lot better today. 我今天身体好多了。(状语)
I learned a lot from him about how to run a band. 我从他那里学到了很多关于经营乐队方面的东西。(宾语)
2. a lot of(=lots of)有形容词性,可用作定语修饰可数名词复数或不可数名词。如:
Ive brought a lot of books back with me. 我带回了不少书。(定语)
A lot of money went on wining and dining prospective clients.不少钱都花在宴请潜在顾客上了。(定语)
▲英语语法网上有一篇很好的文章,可供你参考:
http://www.yygrammar.com/Article/201603/4814.html