Well done. Since this is a known problem and they are working on a fix (they are not actually missing, just the system refuses to show them), we have been trying to add information about them to the Tips & Notes. You really should be reading the Tips & Notes before beginning a skill. You can find them by clicking once on a skill and then clicking on the lightbulb to see them.
This post is from when you could only access the course through a web browser. 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.