whom S3 W1 /huːm/ pronoun
[Language: Old English; Origin: hwam]
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.

REGISTER
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 phrases one of whom, none of whom, some of whom etc.

[TahlilGaran] Dictionary of Contemporary English ▲

whom
pronoun
1.
DUBIOUS: Most of the people whom I met were very kind.
GOOD: Most of the people (that) I met were very kind.

Usage Note:
Whom is used only in formal styles: 'Those applicants whom the selection committee recommends for interview should be contacted without delay.' 'The police officer to whom the crime was first reported has been transferred to another unit.'
In non-formal styles, use that/who or nothing: 'The man (that) she intends to marry comes from Stockport.' 'The girl (that) you were speaking to is a friend of mine.' In non-identifying clauses, however, only who is possible: 'Both girls - who you may remember from Helen's party - have got places at Oxford.'

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2.
BAD: She is one of the few people whom I think might be good at the job.
GOOD: She is one of the few people who/that I think might be good at the job.

Usage Note:
When you need a subject relative pronoun, use who/that (NOT whom ): 'He was the one who/that came to meet me at the station.'

[TahlilGaran] Dictionary of Common Errors ▲