The English language verbs On Guard and Look Out have related meanings, but they are different, as I shall explain, using British English examples.
To Be On Guard means someone who is looking for potential danger or a threat. It is often (but not always) something someone does as a job - for example a security guard or soldier.
"Especially after midnight we have to be on guard for drunks trying to get in."
"Entering the dark alley after midnight, I was on guard for thieves."
"Keep an eye on the water, we need to be on guard for crocodiles."
A Guard (noun) is a person whose job it is to protect something.
"The warehouse guard was sleeping on the job."
A guard can also be an object used to protect something else.
"In front of the fireplace was a fire guard to stop the baby getting too close."
To Look Out means to be attentive to seeing something. This is more general than to be on guard. You can look out for something that is not a threat.
"Please look out for Daisy, I need her to come with me."
Look Out can also be used as an imperative (a command) or a way of getting someone's attention quickly. For example if you saw a car approaching very quickly you might shout "Look out!" to your companion. You can also use it to describe something that has just happened, even if you didn't see it beforehand.
"Look out! You almost hit the kerb."
Lookout (noun) is a place from which you can view the surroundings.
"I posted a guard at the lookout, so we would spot the enemy well before they reached us."
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