It's your birthday. One of your coworkers is giving you a birthday present. You didn't expect to get a gift from her. You want to express your surprise and politely say that she didn't need to get you anything. You say:
Oh, you shouldn't have!
You usually say "oh" when something surprises you, or when you hear new information that you didn't know before:
In the example above, "Oh!" shows that you're surprised at what someone did. You're surprised that your coworker got you a gift. (Or at least you want to act like you're surprised.)
"You shouldn't have!" is a common set phrase that people say when someone gives them a gift. It means "You shouldn't have gotten anything for me!"
Use this expression when you get a gift from someone that you didn't expect. Also, you should know that this phrase sounds a little old-fashioned and cliché. Young people in their teens and twenties might not want to use it with their friends.
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