whom S3 W1 / hum / 代词
the object form of ‘who’, used especially in formal speech or writing :
Desperate for money, she called her sister, whom she hadn’t spoken to in 20 years.
She brought with her three friends, none of whom I had ever met before.
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In everyday spoken or written English, people usually use who rather than whom :
Who did you send it to?
Whom is usually used only in the 短语 one of whom , none of whom , some of whom etc.