Answer 1

It's one of the most important things. Without good knowledge of data structures your algorithms will be unefficient. Try to find some problems which require using of some data structures (I mean not just arrays) and solve them without using those data structures. It is possible that you won't be able to do so, while with using of data structures it is simple task. You want to check brackets matching? Use stack. You need to find element really fast? Use map...
Try also to implement those data structures :)