Translation:the Klingon vowel u
This Sounds unit uses the names of the letters to teach the sounds. All of the other units focus on teaching grammar and vocabulary. We will be redesigning how we will be teaching the sounds in the future, but for now you'll have to learn the names of the letters in Klingon.
I'm not sure what "testing" you are referring to. The teaching model of Duolingo is for you to learn the vocabulary and grammar through example sentences. So the software challenges you by giving you new sentences to learn from.
But if you prefer more of a written lesson explaining things Duolingo has made some of those available, but only through a web browser. If you have not been reading the Tips & Notes, you might find that they help you understand what is going on in the sentences that Duolingo challenges you with.
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 large Start button and a large Tips button.
If you click on the Tips button 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, we are happy to answer your questions, but many of your questions will probably already be answered in the Tips & Notes.