Look at vs. observe

[Shiho]

Test No. express/inter-1 "Up to his ears", question 5

As she is new to the job I would ask you to keep an eye on her for the time being.

(a) look at
(b) consider
(c) check
(d) observe

Test No. express/inter-1 "Up to his ears", answer 5

As she is new to the job I would ask you to observe on her for the time being.

Correct answer: (d) observe

Your answer was: incorrect
As she is new to the job I would ask you to look at on her for the time being.
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Why is "look at" wrong? What exactly is the difference between "observe" and "look at"?

[Alan]

Hi Shiho,

Keep an eye on means watch carefully and that is the meaning also of observe. Look at simply means put your eyes in a certain direction. You can look at a picture but you can observe someone who, for example, is doing something strange and see what happens next.

Alan
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English as a Second Language
You can read my ESL story Present Simple

[Ewy]

Hi,
I made also mistake as I always thought that it's "checking" someone or "watching" to make sure that person won't do something for example stupid.
Why check is wrong ?
Thanks in advance.
Ewy