People do not usually express criticism strongly unless they know the person they are criticizing well.
If you want to criticize someone for doing something badly, you can say something like 'That's not very good' or 'I think that's not quite right'.
What answer have you got? Oh dear. Thirty-three. That's not very good.
I think your answer's wrong.
If you want to criticize someone for doing something wrong or stupid, you can use a question beginning 'Why did you...?' or 'Why didn't you...?' Questions like this can be used to express great anger or distress, or merely exasperation.
Why did you lie to me?
Why did you do it?
Why didn't you tell me?
You can be more direct and say 'You shouldn't have...' or 'You should have...'.
You shouldn't have given him money.
You should have asked me.
Some people say 'How could you?' when they feel very strongly that someone has been thoughtless.
How could you? You knew I didn't want anyone to know!
How could you be so stupid?
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