A: Mary, could you help me with this problem?
B: Sure. Ah, our teacher has explained it in class, hasn't she?
A: Yeah. But I didn't cotton on to what he said.
Cotton on的意思是“理解，懂得，意识到”，它后面往往跟to something。
e.g. I didn't cotton on to your last point, could you explain it again?
最后一点我不明白, 请再解释一下好吗 ?
e.g. She had already cottoned on to the fact that the nanny was not all she appeared.
e.g. It wasn't until he started laughing that they cottoned on!
help someone with something: 帮助某人做某事
e.g. All the neighbours clubbed together to help him with his debts.
e.g. He cheered up at once when I promised to help him with his lessons.