Examples

While on active service, I saw things that I won't forget for the rest of my life.

我在战争中看到了让我永生难忘的事情。

服现役

be on active service → 现役

现役年限

term of active service → 现役

爷爷虽然已经退居二线, 但还经常关心工作上的事。

Though grandpa has stepped down from active service, he still cares a lot about work. → 二线