Klingon is very different from English and at the beginning stages is at least as hard as any language. But as you get used to the differences, it gets easier. If you have not been reading the Tips & Notes, you will find it much easier if you do read them. If you have read them and have questions, please be specific in your question so we can figure out where the confusion is.
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/, either from your mobile device or from a computer. When you click on a Skill, it will expand to reveal a Start button 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.
The meanings of the prefixes bI- and Da- are explained in the tips and notes for this unit.
If you are using the website, you can access the tips and notes by clicking on the little lightbulb next to the Start button after selecting a lesson unit:
Have a look in the "Verbs" section.
If you are using a mobile app, you may not have access to the tips and notes, which makes it more frustrating and confusing to learn Klingon than it needs to be. Consider switching to the website version, at least to open and read the tips and notes before starting a lesson.
If there is something unclear in the tips and notes or something you do not understand about this sentence after reading the tips and notes, please do reply with what you found missing or confusing.
Also, please pay attention to commas in this unit. Just as "Let's eat, Grandma!" means something quite different from "Let's eat Grandma!" in English, so also in Klingon, something like torgh Dayaj'a'? and torgh, Dayaj'a'? mean quite different things.
Because mara is the thing directly before the verb and so must be the object (the thing being understood) and torgh is separated from the sentence (in writing by a comma and in speech by a pause and or change in tone) and so is just calling out someone's name to get their attention.
For a few reasons. Number one is that the Da- prefix on the verb means that the listener is the one doing the action. Dayajbe' means, "you did not understand her".
To say, "Torg did not understand her", we would use the null prefix (in other words, the absent prefix). Torg, the grammatical subject - the one doing the action, would also be placed after the verb. So mara yajbe' torg would be used to mean "Torg did not understand Mara." This would be someone speaking about Torg and Mara rather than directly to them.
Torg, in this exercise, is separated from the sentence by a comma. In this exercise his name is not actually part of the sentence. He is being called before saying the sentence. So now the Da- does, in fact, refer to Torg, but it is because the speaker is actually speaking to him when they say "you".