TIP: Reverse Learning!
I have a friend who also uses Duolingo and he suggested to me that instead of only learning the English > Dutch tree, I should reverse it and also do the Dutch > English tree. I've found this to really helpful as it offers learning from a different perspective. Of course, there are parts were it will say a sentence in English and you have to write it in English but for the most part, it's a really useful thing to do. I would highly recommend it!
I am doing that for Spanish now. However, I would only recommend it for advanced learners because it does not read anything in your original language so you will lose the opportunity to hear the target language.
I do this with Hindi and I am by no means an advanced learner. Someone recommended doing this from the start and I do find it helpful. And for hearing pronunciation, I simply put the target language text in Google translate and click the microphone icon.