It turns a statement into a yes/no question.
An explanation is given in the Tips & Notes. It’s a lot of new information and some details are bound to slip through the memory. But 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 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.
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. If you have questions after reading the Tips & Notes for any Skills, then please return to the forum to ask your question, explaining what you didn’t understand or what seems contradictory to you.
Great! I didn't think to check and see which Skill this is. I just assumed it was the one where we teach that. You are, of course, correct that the -'a' suffix has not been presented yet. You will get a full explanation and plenty of opportunities to practice as you progresses. I'm glad you already know about the Tips & Notes. You'll eventually see all the grammar notes needed to explain the sentences in this Skill.