Can this also be "mara understands him/her/it"? Another example was "mara yaj", meaning "he understands mara".
As I understood, when both object and subject are 3rd person, there is no prefix. So this could be translated as "mara understands it", or is "'oH yaj mara" needed?
That's absolutely correct. When there is no prefix (or, the null prefix, as we sometimes call it) there may or may not be an object. Context will usually make it clear which is appropriate, but in cases where it is not so clear, it is probably a good idea to include a pronoun (like 'oH).
Since this exercise does not include either context or a pronoun, we accept translations with or without an object.
Klingon is very different from English and that can, indeed, make it seem tricky. The Tips & Notes can help to make things less tricky. However, since Duolingo has hidden the Tips & Notes I want to make sure you know about them and where to find them. If you have not been reading the Tips & Notes, you may find it helpful to review those.
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