This is my first time to post a message here....
I don't know when we should use who and whom, anyone can help me?
" He is the guy who made several posts here yesterday "

( who = guy ) is the relative pronoun used as the subject of the clause

" He is the guy whom I met at this forum yesterday "

( whom = guy ) is also a relative pronoun but used as the object of the ' verb ' met '.
"Whom" is gradually being phased out of all but the most formal English. While it's still with us, however, "whom" is the object, and "who" is the subject:
"To whom shall I address this?" "With whom are you going?" "Whom did you shoot?"
"Who wants this package?" "Who was there?" "Who got shot?"
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The word is gradually being phased out? By whom?

The only reason use of the word is in decline is that few people know how to use it properly.

"Long live whom!" (er, maybe that should be a question)
We could get into a long, albeit interesting, discussion about prescriptive vs. descriptive rules of grammar and usage. I'll just say that a balanced approach is usually the wisest: a language cannot live without rules, but the rules should be sensitive to the ever changing nature of language as it is spoken and written.
I think all languages change over time; thus a language of three hundred years ago should be very different from today's language and this seems to be especially so in English, which has been grammatically experiencing big changes for some decades now. As a result, the relative pronouns in the object form like "whom" "which" and "that" are omitted in approximately more than half of the informal sentences with no changing of the meanings.
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I think using "who" and "whom" correctly is still important.

Suppose, for example, you come across the following sentence in a book:

"Rob mentioned who?"

"Who" written here without the "m" indicates that "who" is the subject of the sentence; that is, the speaker does not know "who" did the mentioning. Spoken, it would sound more like "Rob mentioned WHO?"

But were the sentence to read

"Rob mentioned whom?"

then Rob is the subject of the sentence, and the speaker wants to know who was mentioned by Rob.