It is three years since he joined the army.为什么是用joined而不是用have joined

I have been here for three years.=It is three years ______I____here It is three years since she was in the It is three years since he died.(同义句） He has ____ _____ for three years. he has been a soldier for three years.改为同义句：It is three years --------- ---------- ---------the army. my age today is three times what it will be three years from now minus three times what my age was three years ago.how old am we think it possible to learn English well in three years.为什么不是it is possible? it is an o___photo.My father is forty-three years old now It was three years ago that he joined the army与It is three years since he joined the army翻译及区 The length of the road is twice what it was three years ago.这里的what it was three years ago.是不是比较状语从句啊? it is three years since i have seen himit is three years since i saw him有什么不同.表达效果还是意思什么的? t is three years since he joined the League?it is three years since he has joined the League?那个正确?为什么呢? The output of our factory is now three times what it was three years ago.为什么用what Tom has been in China for three years.(改为同义句)1.Tom has been in China ( ) three years( ).2.Three year ( ) ( ) ( ) Tom cane to China.3.Tom ( ) ( ) China three years ( ).4.It is ( ) ( ) ( ) Tom came to China. Tom has been in China for three years.(改为同义句)1.Tom has been in China ( ) three years( ).2.Three year ( ) ( ) ( ) Tom cane to China.3.Tom ( ) ( ) China three years ( ).4.It is ( ) ( ) ( ) Tom came to China. it is three years since he moved here 的同义句转换! It is three years since he became a worker 用现在完成时改写 求It is three years since he worked in his office.解释下since It is three years since I lived here.怎么翻译?为什么这样翻译?