Have you read the Tips & Notes? If you access the course through a web browser, when you click on a Skill, you will see a Tips button above the Start button. Click on the Tips button and you should get all the information you need for that Skill. If you still have questions, please return to the discussions and we'll help you out.
There might be one of two things going on. The most likely is that you are trying to access the Tips & Notes from the app. The Tips & Notes are not available on the apps and you have to access the website through a browser to read the Tips & not6es. If you are trying to access the Tips & Notes through the website, then it may be that Duolingo is not letting you see them until after you have completed a full lesson. You don't have to complete a full crown level just one full lesson. If you have completed a full lesson and are using the website instead of the apps, then I am not sure why you wouldn't be able to access the Tips & Notes.
This exercise accepts "understands", "understood", and "will understand". For third person singular present you should use "understands". Rather than recognize you left off the "-s" from the present tense version, the Duolingo software instead assumed you left out the "will" for the future tense version.
The wordbank offered "does", but didn't like my using it. "Torg does understand Mara" seemed like a good translation, coming immediately after "Mara does not understand Torg".
Is there a difference in Klingon between "Torg understands Mara" and "Torg does understand Mara"?
It's a little complicated. There is a small difference in the English phrases. You would use them in different situations and for a slightly different emphasis. Using "does" seems to give a stranger emphasis, as if you were contrasting against an different expectation and wanted to emphasize a little for that contrast. The example you gave uses it this way. For that situation in Klingon, I would probably add a type 6 verb suffix like, mara yajbej torgh.