Say "He used to be in the army" in Mandarin
Not sure what is the most natural way of saying this. Thanks :)

Question

Answer

You can say 他在部队呆过。

Other answers are good . My answer is short, so maybe it is easy for you to say .

He -  他
Used to be - 呆过（our difference is that we translate these words in different way)
In the army - 在部队

Answer

他以前在军队呆过
If he was a soldier, you can say
他以前是军人

Answer

他以前在部队。

Answer

This translation below is a bit more formal:
他以前在部队服役。(  He served in the army.)

Answer

他以前在军队里