Be on your guard against pickpockets.

提防扒手。

We should learn to guard against some illnesses from a young age.

我们应该从小就学会提防一些疾病。

At the outset, it seemed necessary to be on guard against the other two.

一开始，似乎有必要提防另外两个。

Security cameras have been installed to guard against poachers.

已经安装了安防摄像机来防范偷猎者。

The armed forces were on high alert to guard against any retaliation.

武装部队保持高度警惕，以防发生任何报复行为。