1.He added that to be safe, people should always be on their guard when communicating with a stranger online.
on their guard警惕
They did their best to keep on their guard [ on the alert ] .
他们尽量保持警惕 。
2.This is all that is needed to pretend to be someone looking for love. The criminals pose as potential partners and contact people seeking romance on dating sites.
pretend to be假充;冒充
The children pretend to be different animals dancing to the music.
孩子们扮成不同的动物,随着音乐起舞 。