Errors connected with the use of this word are very common, even
amongst good speakers.
"Who did you see?" "Who do you know?" and "Who did you hear?" are
wrong: _whom_ should be used, for it is the object of the transitive
verbs, _see_, _know_ and _hear_. _Who_ in such sentences as: "Who are
you looking at?" and "Who are you writing to?" should likewise be
changed into _whom_, for it is the _object_ of the prepositions _at_
and _to_.