No -- Mara is not the subject.
In fact, she's not "part of" the sentence at all: not the subject of the verb, not the object, not a recipient -- she's not involved in the action at all.
It's just a vocative, to draw the listener's attention to the fact that you are speaking specifically to them.
As in "Hans, where did your father go?"
The subject is "your father", and the "Hans" at the beginning just calls your attention or specifies that I am talking to you and not to Jeremy.
This kind of vocative is set off by a comma and can appear at the beginning or at the end of a sentence.
This seems excessively tricky to put into a beginning sentence. I translated it as "Does Torg understand Mara?" and was informed that the correct answer is "Mara, does Torg understand her?" If, as you say, Mara is a vocative, then how do I deduce that there is a "her" somewhere to be understood? What indicates that it is specifically a "her" and not a "him?" Would removing the comma make the sentence "Does Torg understand Mara?" Please elucidate.
Klingon verbal prefixes indicate the subject and the object.
The null prefix (when there is no prefix at all) indicates that the subject is third person (he, she, it, them) and the object is either nothing or is third person (him, her, it, them). (*)
So yaj'a' torgh? can mean "Does Torg understand?" or "Does Torg understand him?" or "Does Torg understand her?" or "Does Torg understand it?" or "Does Torg understand them?" -- and all of those translations are accepted for that part of the sentence.
There is nothing in this sentence to indicate that there must be an object (the null prefix can also be used when there is no object) nor that the third-person object is specifically "her".
To answer your other question: yes, mara yaj'a' torgh? without comma does mean "Does Torg understand Mara?"
(*) Exception: the combination of subject = they and object = him/her/it needs a prefix lu-.
While the system showed you "Mara, does Torg understand her?" as the correct translation, it will also accept and sometimes show "Mara, does Torg understand?", "Mara, does Torg understand him?", "Mara, does Torg understand it?", and "Mara, does Torg understand them?" It's not trying to trick anyone it's trying to teach this exact concept.
Consider the differences between these two English sentences:
"Let's eat, Grandma!"
"Let's eat Grandma!"
The first one is talking to your grandmother and suggesting a shared meal. The second one is advocating cannibalism.
Duo's sentence here uses the comma in a similar way, to show whom you are talking to.
mara, yaj'a' torgh? = Mara, does Torg understand?
mara yaj'a' torgh? = Does Torg understand Mara?
Notice the comma! You have the correct translation of this sentence for when there is no comma. This sentence is showing how to call someone's name and then say your sentence. The comma indicates that Mara is not part of the actual sentence, she is just the person you are asking the question to.
There should be a comma and a question mark and then it makes perfect sense. See how it is written at the top of the sentence discussion? When Duolingo grades your answer, it ignores punctuation, so it's possible to enter the translation without punctuation. Also when Duolingo gives you a tile exercise, it usually just gives you the words and not the punctuation. This is true for all courses on Duolingo.
"Mara, does Torg, Understand her?" for the right translation. It in my head sounds like "The rock wants says this..." (While the popular person rants about something),Is that a bug? Or is that a take it in a conversation thing? For example two people arguing about a misunderstanding, and someone asks: More like: Does Mara Know if Torg understand a (Dax)?
yaj'a' has an empty verb prefix (there is nothing before the verb stem yaj).
The empty prefix means that the subject is third person (he, she, it, they) and can mean that there is no object or that there is a third-person object (him, her, it, them).
So yaj'a' torgh? can mean any of "Does Torg understand?" (no object) or "Does Torg understand him?" / "Does Torg understand her?" / "Does Torg understand it?" / "Does Torg understand them?" (third-person object). All of those translations should be accepted.
Adding mara, ... (with comma) to the beginning doesn't change the possible interpretations of yaj'a' torgh?.
That is very odd indeed. I assure you that in the database it has both a comma and a question mark. We have no control over the weird contortions that the Duolingo software does to the sentences that we have entered into the database. If you see that again, please take a screen shot and post it in the forums and/or send in a bug report.
'Mara does Torg understan' doesn't make any sense...
No, because you left out the comma.
"Mara, does Torg understand?"
You are talking to Mara and asking her whether Torg understands.
You call her by name so that she will know that she is being spoken to, and then you add your question.
"Mara, does Torg understand?"
mara, yaj'a' torgh?
There is a learner comment on this sentence, "English translation is not English." We have no way of knowing what form of question this learner received, nor what translation they saw. It's an odd-sounding error, but definitely let us know--a screenshot would be great--if you get a "translation" that isn't in English or doesn't make sense in English.
I suspect this complaint is a result of someone seeing Mara does Torg understand as reported by bobruben a year ago, and Julia348743 two weeks ago, so I'm going to leave the complaint uncleared, as something we can perhaps report to Duolingo as a problem.