3 sentences

1. 警察正在街上执勤。

   jǐng​chá zhèng​zài jiē​shang zhí​qín.

   The police are on duty on the street.

   Source: Dong Chinese Dictionary

2. 这名警员在执勤时表现得非常专业。

   zhè míng jǐng​yuán zài zhí​qín shí biǎo​xiàn de fēi​cháng zhuān​yè.

   The officer acted very professionally while on duty.

   Source: Dong Chinese Dictionary

3. 指挥中心正在呼叫正在执勤的巡逻车。

   zhǐ​huī zhōng​xīn zhèng​zài hū​jiào zhèng​zài zhí​qín de xún​luó​chē.

   The command center is calling the patrol car on duty.

   Source: Dong Chinese Dictionary