"Those who have a heart don't have to teach, and those who have no heart can't teach", which means that those who have a heart will teach themselves off-site even if they don't have to teach everything, and those who have no heart can't learn even if they work hard every day.

If a person's mind is focused on learning, he can learn everything well. If he doesn't want to learn, no matter how well the teacher teaches him, he can't learn. It depends on whether he concentrates on his studies.

And some people learned it by accident, but they gained a lot.

It means that a person's talent is very important. Even if he studies something, he doesn't care and has no intention to understand it, but he gets the answer by accident, or he can easily understand the meaning.