Han var i armén under trettio år.

[davidculley]

English Translation

He was in the army for thirty years.

Why is it “under trettio år” and not “i trettio år”?

[morbrorper]

“I trettio år” is better. “Under” is even a bit ambiguous, as it could mean “less than”. You would use “under” in “Han var i armén under trettioåriga kriget”.

An alternative translation is “Han var trettio år i armén”.

[davidculley]

Thank you so much @morbrorper for your detailed and very helpful answer