Which vs. who

[Mister Micawber]

To whom did you give the roses?-- Formal
Who did you give the roses to? -- Informal
Whom did you give the roses to? -- Neither fish nor fowl; generally frowned upon.
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Native English teacher at Mister Micawber's

[Niaz1]

so how could we understand do we have to use whom or who?

[Mister Micawber]

Just use them as steelnlace has already explained in post #14.
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Native English teacher at Mister Micawber's