This one is a little strange. You have 混じる (to get mixed up in), 混ぜる (to mix), 混ざる (to get mixed), and now this: 混む, to be crowded. It makes sense that when everyone's all mixed up it's going to be crowded, but how to differentiate between this and the other 混 words? Just think of how cows muuuuu (む) get all bunched up and crowded, which is why this word is to be crowded.
Hey, come (こむ) out of that crowd of cows. It's going to be crowded in there.
The train is crowded every morning, but it’s not your fault. There’s no need to blame yourself.