There are sentences about learning in The Analects of Confucius. Write their meanings.

1, review the past and learn new things, which can be used as a teacher: review the past and learn new things, which means to know and judge new things through understanding and mastering known things;

2, sensitive and eager to learn, not ashamed to ask questions: smart and eager to learn, not ashamed to ask people who are lower in status and less knowledgeable than themselves;

3. Learning without thinking is useless, thinking without learning is dangerous: reading blindly without thinking will make you unable to use the knowledge of books reasonably and effectively because you can't deeply understand the meaning of books, and even get confused;

4. Knowing means knowing, not knowing means not knowing, knowing means knowing, and not knowing means admitting not knowing. This is true wisdom.