她来我们学校三年了.可以这样说：
It is three years since she came to our school.也可以这样说：
It has been three years since she came to our school.
说明：1）since 是连词,汉译是“自从...以
来”.
2）从过去一直延续到现在的动作用现完
成时,时间状语用for...或since...
3）come是瞬间动词,不可以和表示一段的
时间状语连用.然而我们想表达“来了
一段时间了”这个意思,就改变说法,
可以换词（就是把come 换成be in)
也可以换句型,可以换成it is(has
been)+时间+since+主语+瞬间动词的过
去式.
那么“她来我们学校三年了.”可以翻译成：1.She has been in our school for three years.2.It is (has been)three years since she came to our school.3.Three years has passed since she came to our school.4.She came to our school three years ago.但不可以说：She has come to our school for three years.