be afraid of 是以动词、形容词和介词组成固定的表达，在你说的句子里不能省略介词of，用来连接害怕的对象（car)这个宾语。同义的短语还有be frightened of, be scared of你不想用含介词的短语，就用同义的及物动词，比如：Were you afraid of the car? = Did you fear the car?...
afraid是个形容词，而且是句子中的表语，所以后面不能直接跟名词，所以要用of。下面的句子都是对的：Were you afraid?Were you afraid of the car?Were you afraid of driving the car?of是介词，用的话就要跟上名词或者动名词～...
