Одні is plural, you would use it with the plural only nouns: одні штани (one pair of pants), одні окуляри (one pair of glasses)
Am i missing something about Duolingo? Are these things explained somewhere? I feel like we just get thrown in to each session without any prep.
There are Tips and Notes. When you open the skill, scroll down below the lessons and there are tips and notes. Or when you are in a lesson, you can click and there will be a drop down box that has tips and notes.