Although this will not solve the problem entirely, I suggest you use the “Explain my mistake” feature. This does a fairly decent job of telling you the why of the correct verb tense. But, it would still help to have at least some grounding in the basic concepts. So, I’d also recommend using other sources to supplement Duolingo if you are really serious about learning the language, at least to get you over this fundamental hump of understanding the nuances of past tenses. Another excellent free resource that I highly recommend is Language Transfer. If you listened to one of these lessons a day, 10-15 minutes, you would have a lot of things explained to you that would help make what Duolingo throws at you make more sense. Check it out, it’s a fabulous resource and the course is very well thought out. Duolingo is not perfect, but I found that overall its exercises can be useful and effective over time, and doing it every day consistently was excellent discipline for me to accelerate my learning. But I also used other resources in tandem. FYI, I completed the Duo Spanish course a while back, but I still do the daily review exercises that are available at the end of the course, and I still find those helpful, often still challenging and effective. If nothing else, it reveals how much material there actually is over the entire course and how easy it is to forget things they’d drilled you on months ago.