Duolingo has brief explanations of the grammar for each unit. It's not even close to a textbook by any means but it's not impossible to learn. Now that I think about it, my mental algorithm works like this:
1. Observe the words
2. Observe the order they were used in
3. Have their meanings in english as reference
4. Use this information to puzzle out the meaning of the sentence
This is pretty much the same process I used to learn english when I was a kid. Only I used video games and natives on forums instead of an english course.