We need to stay on guard.

我们要保持警惕。

Teachers must stay on guard when it comes to what their students are doing in classrooms and how technology could be playing a negative role in the learning process.

教师必须保持警惕，了解学生在课堂上的一举一动以及科技如何在学生学习过程中起负面作用。

While U.S. Defense Secretary Mark Esper called al-Baghdadi's death a devastating blow to ISIS, America and other nations said they'd stay on guard against the next terrorist leaders who appear.

虽然美国国防部长马克·埃斯珀称巴格达迪之死对ISIS是一个毁灭性的打击，但美国和其他国家表示，他们还是会对下一个出现的恐怖组织头目保持警惕。