How could you? means “How could you do such a thing?”. It isn't a question about method but an expression of outrage or hurt: the person did something nobody expected of them. It sounds emotional, so it fits conversations with people close to you, not a business meeting.
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How could you?; How could you + verb…?; How could you not + verb?; How could you know? / How could you have known?
- You read my private messages while I was in the shower? How could you?
- How could you leave the dog alone in the car on such a hot day?
- How could you not tell me you were moving to Berlin?
What changed
First versionHow you could forget my birthday? I reminded you yesterday!
RevisionHow could you forget my birthday? I reminded you yesterday!
The order you could is how a statement is built, and it's easy to keep it in a question. But this is a question, and in questions could comes before you. Nothing else was changed.
Add know, and the reproach becomes comfort: I didn't know the shop closed early on Sundays. — How could you know? They only changed the hours last week. How could you know? or How could you have known? means “there was no way you could have known”. How can you…? is about what someone does in general or right now: How can you eat that?
Your turn
You promised to come to my party and didn't even call — how ___ forget?
Show the answer
could you — This is a question of reproach, and in a question could comes before you: how could you forget. The order you could appears in a statement (I know you could help) or after I don't know how, but not in a direct question.
A friend has told your secret to people you both know. Write her a message with how could you and say what exactly she told them.
How could you tell everyone at work about my new job before I'd even signed the contract?
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