Definition of how can/could
1 —used to show that one thinks that someone has done or said something shocking or wrong "We don't need his help anyway." "How can you say that?!"How could she just walk away from her children like that?
2 —used to express doubt that something will happen, is possible, etc. How could I (ever/possibly) leave this job?How can I (ever/possibly) thank you?
3 informal —used to show disappointment in someone's actions, thoughts, words, etc. He threw away my gift? Oh, how could he?