【初中·阅读理解·Learning from Mistakes】
When I was in Grade Seven, I failed my first English test. I had spent the weekend playing games instead of learning the new words.
My teacher did not shout at me. She asked me to stay after class and handed me a small notebook. "Write down every mistake you make," she said. "Not the correct answer — the reason you got it wrong."
At first I thought it was a waste of time. But after a month the notebook was full, and I could see my own habits: I always forgot words with double letters; I always mixed up the past forms.
Three months later I came top of the class. The notebook did not make me cleverer. It simply showed me what I was really doing.

第 3 题：What is the main idea of the passage?（　　）