[误] I have known him since a child.

[正] I have known him from a child.

. I have known him since his childhood.

注：从过去某一时期到现在为止，英语要用现在完成时态，介词就用 since，但有时不说“迄”(till)也可以说“起”(from)的，即用“from + 普通名词”代替“since + 抽象名词”。但在 from 后也同样可接抽象名词，所以不说 He has been blind since childhood，而说 He has been blind from childhood. 也是一样。