First of all, in casual spoken English a lot of people only use "who". So if your main goal is to sound natural when speaking, forget about "who": Who did you see? Who did you send it to?
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In more formal English, you use "who" as the subject and "whom" as an object:
Who did you see?
Who did you send it to?
Who did she marry?
Who did you see?
To whom did