"I don't understand, Mara."
jI- indicates that there is no object (or at least a general, non-specific object), so things like, "I sleep", "I am happy", "I eat" (notice that I must eat something, but by using jI- I am saying that it is general and I'm not talking about eating something specific). So in this sentence it is saying that the not-understanding is general and not that there is something specific that I don't understand.
vI- indicates that there is a specific object (or objects) I have in mind, whether I have stated it (them) or not. So things like, "I eat it", "I hate them", "I want a weapon."
If I do want to specify what it is, I have to include it in the sentence in the object position: nuH vIneH ("I want the weapon.") But if I think you already know what I'm talking about, like if we have already been talking about the weapon, I don't have to put it in the sentence: vIneH ("I want it.") You will learn later about pronouns, which can be added to the sentence (like the "it" in the English translation). In English such pronouns are required (you can't say "I want" as a proper English sentence if you have a specific thing in mind), but in Klingon they are optional, so the prefix tells you whether to assume a specific, but unstated object or not.
An explanation is given in the Tips & Notes, but I know it’s a lot of new information and some details are bound to slip through the memory. 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, I would like to ask that you review those so we don’t have to repeat too much of 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 question, explaining what you didn’t understand or what seems contradictory to you.
Don't you do the same in English? Sometimes you want to call the person's name first to make sure you have their attention: "Mara, I don't understand." Sometimes you say what you wanted to say and then decide you should call the person's name so they know you are talking to them: "I don't understand, Mara." The same thing happens with Klingon speakers. And when translating from one language to the other, we ask that you keep that order. If one language calls the person's name first, then the translation into the other language should also call the person's name first. Or if one language calls the person's name after, then the translation into the other language should also call the person's name after.
I completely agree. We are more strict in Klingon mostly because there is a tendency for students to just try to do everything in reverse in Klingon. Many aspects of Klingon grammar present things in reverse order from how they appear in English - but not all aspects! Because of this many students get used to reading and writing sentences from the back and treat the language as if everything should be written in reverse order. We want to break this habit from the very beginning and help students learn when the order must be reversed and when it can be kept the same. Thus we are very strict on only changing the order when the grammar calls for it and insisting that things remain in the same order when the grammar allows a similar flexibility in both languages. It is a pedagogical technique specific to Klingon and we understand that it can be a little frustrating at first, but we feel that overall it leads to better Klingon learners.