keep an eye on

(verb) to watch someone carefully, to take care of someone or something

Example Sentences:

Can you keep an eye on my boyfriend while I’m on vacation? I don’t want him to cheat on me.

Carol asked me if I would mind keeping an eye on her houseplants while she was in Cuba.

The nervous babysitter kept an eye on the poorly-behaved children.

Similar idioms:

1. see eye to eye
2. have something on one’s mind
3. tell on someone