为了安全，你骑自行车时必须戴安全帽。

Wèile ānquán, nǐ qí zìxíngchē shí bìxū dài ānquánmào.

For safety, you must wear a helmet when riding a bicycle.

你必须按时交作业。

Nǐ bìxū ànshí jiāo zuòyè.

You must hand in your homework on time.

进入图书馆必须出示学生证。

Jìnrù túshūguǎn bìxū chūshì xuéshēngzhèng.

You must show your student ID to enter the library.