Translation:I don't understand.
This sentence discussion is probably a good place to have an explanation of how to find the Tips & Notes. The original concept behind Duolingo was that learners would pick up the languages only through example sentences and without grammar explanations. The have now expanded to more complex languages where some instruction is going to be necessary, but they still have the Tips & Notes hidden. 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.
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Klingon does not have contractions, but there is no reason that you can't use contractions in your English translations.
It used to be that people learning English we taught that contractions were too informal and that they never use them for fear of seeking too casual. Even still, today, many English learners find contractions confusing. Because of these reasons it is a stereotype that people who have learned English as a second language avoid contractions.
I'm sure that the producers and writers on Star Trek took advantage of that and refused to let Worf use contractions as a subtle way to imply his foreigners. Same with Spock in The Original Series.
This is not the national language of any country. There are speakers spread around the world in many countries. This is a language which was constructed for use by the fictional Klingon alien race in the Star Trek movies and TV shows. It is a real language with real structure and extensive vocabulary, but it does not actually have any native speakers and the practical uses are extremely limited, so it's mostly studied as a hobby. Many of the language's top speakers speak multiple natural languages and have very complicated and useful jobs, but want to just do something fun and useless from time to time. If it sounds interesting, check it out. If not, thanks for asking - I hope you've learned something today.
Oops. I misread your question as between vI- and jI-. Use bI- when the one doing the understanding is the person you are talking to and you want to be general and not refer to some specific thing. bIyaj "You understand."
There is also Da- for when the person doing the action is the person you are talking to and you want to specify something specific. Dayaj "You understand it."