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I need a better understanding of when to use who vs whom. I also have a couple of other usage questions. I have always referred to Merriam Webster online for general spelling questions. Recently, I have discovered a need to understand more detailed grammar rules. Can anyone recommend a web site for quick questions?
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Who is used for subjects. Whom is used for objects.

There is a grammar section of this website for grammar questions. I also recommend getting a grammar book, the one I use is by Diana Hacker: http://www.amazon.com/gp/product/0312412622/sr=1-1/qid=1152992107/ref=pd_bbs_1/102-4479943-744493...
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There is also an excellent discussion on the usage of Who and Whom provided by American Heritage Dictionary:

Usage Note: The traditional rules that determine the use of who and whom are relatively simple: who is used for a grammatical subject, where a nominative pronoun such as I or he would be appropriate, and whom is used elsewhere. Thus, we write The actor who played Hamlet was there, since who stands for the subject of played Hamlet; and Who do you think is the best candidate? where who stands for the subject of is the best candidate. But we write To whom did you give the letter? since whom is the object of the preposition to; and The man whom the papers criticized did not show up, since whom is the object of the verb criticized. · Considerable effort and attention are required to apply the rules correctly in complicated sentences. To produce correctly a sentence such as I met the man whom the government had tried to get France to extradite, we must anticipate when we write whom that it will function as the object of the verb extradite, several clauses distant from it. It is thus not surprising that writers from Shakespeare onward should often have interchanged who and whom. And though the distinction shows no signs of disappearing in formal style, strict adherence to the rules in informal discourse might be taken as evidence that the speaker or writer is paying undue attention to the form of what is said, possibly at the expense of its substance. In speech and informal writing who tends to predominate over whom; a sentence such as Who did John say he was going to support? will be regarded as quite natural, if strictly incorrect. By contrast, the use of whom where who would be required, as in Whom shall I say is calling? may be thought to betray a certain linguistic insecurity. · When the relative pronoun stands for the object of a preposition that ends a sentence, whom is technically the correct form: the strict grammarian will insist on Whom (not who) did you give it to? But grammarians since Noah Webster have argued that the excessive formality of whom in these cases is at odds with the relative informality associated with the practice of placing the preposition in final position and that the use of who in these cases should be regarded as entirely acceptable. · The relative pronoun who may be used in restrictive relative clauses, in which case it is not preceded by a comma, or in nonrestrictive clauses, in which case a comma is required. Thus, we may say either The scientist who discovers a cure for cancer will be immortalized, where the clause who discovers a cure for cancer indicates which scientist will be immortalized, or The mathematician over there, who solved the four-color theorem, is widely known, where the clause who solved the four-color theorem adds information about a person already identified by the phrase the mathematician over there. · Some grammarians have argued that only who and not that should be used to introduce a restrictive relative clause that identifies a person. This restriction has no basis either in logic or in the usage of the best writers; it is entirely acceptable to write either the man that wanted to talk to you or the man who wanted to talk to you. · The grammatical rules governing the use of who and whom apply equally to whoever and whomever.

Excerpted from The American Heritage® Dictionary of the English Language, Third Edition © 1996 by Houghton Mifflin Company.
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Comments  
Who is a subject pronoun and whom is an object one.

I like you.
'I' is the subject and 'you' is the object.
Who likes you?
Whom do I like?

According to modern English where whom is correct, who is correct too, except in this case:

I like to be with you.
With whom do you like to be?

Who do you like to be with. This sentence is a disaster for consevative linguists for two reasons, however, deemed correct according to modern English:
Who is Whom and prepositions should never go to the end of the sentence.

I hope my comments will not unleash a torrent of abuse from anybody.
 Likeguslee's reply was promoted to an answer.
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I strongly suggest to make a search at this site (the search box in the top right corner) with:
who whom

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