"Does Mara understand?"
It's not actually that hard, you're just not used to it.
The order of a basic sentence is object-verb-subject. The object is the noun to which the action is done. The subject is the noun which does the action. Not all sentences have objects. Klingon allows pronouns (he, she, it, they, etc.) to be dropped from object or subject positions, so sometimes it looks like there is no subject or object when it's just an elided pronoun.
In this case you have a basic sentence with a verb and a subject — there is no object. The verb is yaj understand, and it's got the suffix -'a' on it, which makes it a yes/no question. The subject of the sentence is mara, which is a Klingon's name.
So it's a question as to whether the subject, Mara, does the verb, understand.
Since Duolingo has hidden the Tips & Notes I want to make sure you know about them and where to find them. They will be a big help in trying to understand what is going on. If you have not been reading the Tips & Notes, I would like to ask that you review those so we don’t have to continuously repeat the information that we have explained there.
If you are doing the course on iOS or Android, you cannot currently access the Tips & Notes through the app. To access the Tips & Notes, you will have to access the course using a web browser at https://www.duolingo.com/. You can still do it on your mobile device, but you will have to use the web browser instead of the app (or you can do it from a computer). When you click on a Skill, it will expand to reveal a Start button, a key, and a light bulb.
If you click on the light bulb it will reveal the Tips & Notes and give you a detailed explanation of the grammar that is introduced in that Skill. If you have questions after reading the Tips & Notes for any Skills, then please return to the forum to ask your questions, explaining what you didn’t understand or what seems contradictory to you.