> When learning something, when you simply cannot grasp a concept however hard you try, what's the course of action?

Learn by doing, don't learn by studying.

Pick a (hopefully real) problem, solve it using classes. Repeat. Continue repeating. You will either learn what OO is good for, what it is not good for and how you can use it to write better code. Or, learn that programming is not something you can grok.

If you don't have a "real" problem; Then write program to play tic-tac-toe; manage the and display board, take player inputs, detect when game is done (winner or draw).

Then expand it to 8x8 board, then to 3 players. Those changes should be easy with good OO design (lots rewrites, code changes). And probably "harder" with bad OO or no OO design.

btw this is 2nd interview question I and my team used for years.