If you want to understand DAX, sooner or later you’re going to have to deep dive into Table Functions. The SQL BI guys Marco Russo and Alberto Ferrari train people in Table Functions before CALCULATE. And for good reason.
If I’ve learned anything about DAX, it’s that you should start simple to understand complex concepts. I did that in these videos, which I hope helps.
Both of these videos are based roughly on training examples from the SQL BI group’s DAX training. If you can go to that training, do. It’s great. If you can’t go to the training, go to their site or read their book, DAX: The Definitive Guide by Marco Russo and Alberto Ferrari.