You mustn't speak loudly in the library.

你必须在图书馆里大声说话。（禁止大声说话）

同一单元的其他句子

• You're supposed to shake hands when you meet someone.当你遇到某人时，你应该握手。
• In Japan, you're supposed to bow instead of shaking hands.在日本，你应该鞠躬而不是握手。
• Are you supposed to kiss on the cheek in France?在法国，你应该亲吻脸颊吗？
• We're not supposed to arrive late for the meeting.我们不应该开会迟到。
• What are you supposed to do when you enter someone's home?当你进入别人家时，你应该做什么？