I got some plastic cord (which I assume is polypropylene) that has been badly stored that when unravelled it tries to bend back to its mangled state.

I want to practise knots but it's really twisted and I don't know how to straighten it.

Are there any tips you'd recommend? Or should I just buy a new one?

Another question: How do you tell one kind of rope from another, and which kind do you think is the best for practising or for the outdoors?