You are of great help.中的of great help是否修饰you，而句子把它省了
热心网友
没有省啊，这里of great help=helpful就是you are helpful一般来说of+名词 相当于这个名词的形容词，但不是每个名词都是这样，另外用得比较多的是importance例如：it is of (great)importance to study english well.可以改为：it is important to study english well.