他来了三天了 it is three days since he came it has been three days

it is three days since he came.

it has been three days since he came.

1. It is some time(一段时间) + since引导的时间状语从句 = It has been some time(一段时间)...

2. since引导的从句谓语为一般过去时,而主句是一般现在时或现在完成时.
3. 该从句的谓语动词是“瞬间性动词”时,从句正常翻译；
而如果该从句的谓语动词是“延续性动词”作,从句必须作否定翻译.
如：It is two hours since he left the classroom.
自从他离开教室以来,已经两个小时了.
It is three years since he worked there.
自从他不在那儿工作以来,已经三年了.

希望满意,祝您进步：）

It is 3 days since he came.
他来了三天了。
It has been 3 days since he came.
从他来已经过去三天了。
其实意思差不多，只是两种不同表达，前者是一般现在时，后者是现在完成时。

我觉得通俗点说应该是：
IT has been three days since he came here等于it is the third day since he came
it is three days since he came here等于he came and stayed here for three days
区别
前面是他来了三天，说这话的时候可...