If you are using the Duolingo app, for more Grammar explanations I suggest checking the Tips & notes section under the lessons on the website. They are available on the desktop (not mobile) website only. Also, check the sticky notes with the resources, there are tons of information out there! :)
Depends on the grammatical case and number. In the singular number, most nouns that end in -α, -η are feminine, whereas nouns that end in -ος, -ο are masculine (-ος) or neuter (-ο). But anyway, this is something you're going to figure out yourself after looking at more examples, it's not as easy to explain it in a single comment. Make sure to read the tips & notes of each skill on the web version!