Question

he joined the army last year 改成现在完成时

Answer 1

我觉得应该是 he has been in the army for a year.
因为join 是非延续性动词（也就是短暂性动词），与die, borrow , ,appear,begin,borrow,lend,buy,close 等都是表示一瞬间的动作，并不可以与表示一段时间的状语连用。
你不能说加入这个军队就整整加入了一年，而是完成了那个一瞬间的动作（比如举行一个仪式或者签订一个东西什么）就代表你已经加入了军队，到后来就只能说你呆在军队一年了（这是一种状态），所以如果改成完成时要用 has/ have been in 。

或者你还可以用
“It has been/ It’s +时间段+ since从句（动词用过去式）”
“时间段+ has passed + since从句（动词用过去式）”
这两个句型来把这个句子改成完成时，那就可以用join了。

It has been a year since I joined the army.

A year has passed since I joined the army.

希望对你有帮助。

Answer 2

He has been joined the army since last year

Answer 3

he has joined the army last year

Answer 4

he has joined the army

Answer 5

He has joined the army for a year.

若对您有所帮助，请采纳。

追问

为什么不是He has been inthe army for a year.呢

追答

也可以的……意思一样……