"I do not understand Mara."
This is touched on in the Tips & Notes, but I'm not sure whether it's that the Tips & Notes did explain it well enough or that you didn't see the Tips & Notes. 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 read 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.
The tips and notes for this unit are at https://www.duolingo.com/skill/kl/Useful-phrases/tips-and-notes.
Note also the spelling: both the prefixes jI- (subject = I, no object) and vI- (subject = I, object = him/her/it/them) have a capital i, not a small L.
The capital i I is a straight line in the font used on Duolingo (at least on the website), while the small L l has a little curl at the bottom.
mara is the object of the verb. It is a name, and names are third-person nouns. Since there is only one name, it is a singular third-person noun. Therefore, you need a verb prefix that indicates jIH I subject and mara (singular, third person) object. That prefix is vI-.