1. Tell the truth: Tell the other person directly why you don't want to go to the library, such as you have other plans or don't want to go out.
2. Make suggestions: If your study efficiency is not high or there are other reasons, you can make other suggestions, such as studying in the dormitory or going to the library another day.
3. Thank each other for the invitation: Even if you don't want to go to the library with each other, you should thank each other for the invitation and show that the invitation is out of respect for you.
In short, when you refuse, be polite and sincere, let the other person understand your ideas, and at the same time, respect the other person's ideas.