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In the sentence "You changed the name! Oh I'm so proud. Thanks to whomever was responsible for this!" Is whomever or whoever the correct choice?
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It should be "whoever"
The case is determined by the subordinate clause, not the main clause.
Thanks to whoever was responsible for this!"
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To determine whether to use whoever or whomever, here is the rule:
him + he = whoever
him + him = whomever
Examples: Give it to whoever/whomever asks for it first.
Give it to him. He asks for it first.
Therefore, Give it to whoever asks for it first.
We will hire whoever/whomever you recommend.
We will hire him. You recommend him.
him + him = whomever
We will hire whoever/whomever is most qualified.
We will hire him. He is most qualified.
him + he = whoever

Hope the above helps.
Thank you!