"In order to build this temple, they used what appears to be bones."
Translation:chIrghvam chenmoHmeH HommeyHey lo'lu'pu'.
English doesn't have a distinct indefinite subject, so you wouldn't see it in the English. The writer of this sentence apparently means to use they as an impersonal pronoun. It doesn't matter exactly who they are.
English also doesn't mark verbs for perfective. Perfective means an action is described as a completed whole. That's what's happening here: they used indicates that the action of using something was performed and completed.
Usually, this course incorrectly equates Klingon perfective (-pu', -ta') with English perfect tenses (have done, will have done, had done). English perfect tenses can often be translated using Klingon perfective, and vice versa, but the two mean different things. Where perfective means an action is described as a completed whole, perfect tense means the action is done BEFORE some time.
It would be correct to translate this Klingon sentence with they have used, but it's also correct to just say they used.