2 sentences

1. 他察觉到身后的脚步声，立刻警觉起来。

   tā chá​jué dào shēn​hòu de jiǎo​bù​shēng, lì​kè jǐng​jué qi​lai.

   He became alert as soon as he noticed footsteps behind him.

   Source: Dong Chinese Dictionary

2. 面对复杂的局面，我们必须保持高度的警觉。

   miàn​duì fù​zá de jú​miàn, wǒ​men bì​xū bǎo​chí gāo​dù de jǐng​jué.

   We must maintain a high state of alertness in the face of a complex situation.

   Source: Dong Chinese Dictionary