Previous lessons should remain the same
I have been using Duolingo for about a month now and find that when i want to revise what i have done in the lessons i have completed, they have moved forward to what i am up to. It would be great if they remained the same as when you complete them initially as it would make revision a lot easier to focus on particular groups of words
That's the beauty of Duo! Initially I didn't like how they integrated the materials learned in latter lessons into earlier lessons because they take longer to do and I end up with a bunch of errors. Now that I'm about 3/4 of the way thru the tree I'm finding that I'm learning so much more because the categorization boundaries are getting more fluid, more real life like. Yes, revisions would be a lot easier to focus on but, in the end, you have to use words in multiple contexts if you hope to engage in conversations with native speakers. I don't think that it is particularly useful to just memorize words / sentences in some stagnant arbitrary grouping.
I agree with Cheryl1. The farther you go in the tree; the more I appreciate how all of the lessons adjust. When I review Animals, I'm likely to have to use past tense which isn't introduced until much later in the tree. Although I'd still be reviewing Animals, but it's pretty much all new sentences.