i have known him____he was a boy of eight years old

问题描述:

i have known him____he was a boy of eight years old
为什么不能用when

since
一般在现在完成时中用since引导过去时的分句.
既然前面用了完成时,就表示一段时间以来,而后面要是用when的话就只是表示认识他的那个时候,而并不能表达一直到现在的意思.