Spoken English:
I couldn't agree (with you) more.
Use for emphasizing that you agree with something that someone has just said.
"more" 是比較級的 adverb 用來修飾 "agree" 這verb.
意思是：我不能同意得再多了.那就是：我非常同意.
Example:
A: This will satisfy everybody.
B: I couldn't agree more.
慣性地使用couldn't : simple past 時態(tài). 因為所同意的事,是指A剛才說了的事.
I can't agree with you anymore.
這句中,"anymore"的理解最重要.
Anymore (adverb): means no longer, any longer再也不 的意思
Anymore 不是比較級adverb,沒有再多的意思
是故通常這樣用：
I don't live here anymore.
I will not agree with you anymore.從今再以后不再同意你的做法(以前常常認(rèn)同)

因為anymore 的字意,故不會說
I can't agree with you anymore.(這是文法正確,但意義不明的英文句子.)
Just say: I can't agree with you./ I don't agree with you.
I can't agree with you ANY MORE.
注意：Any more 要分開.這樣more才有比較級adverb的意思
Example:
He doesn't want any more = He doesn't want more.
Any 修飾 more,可以除去卻不影響句意
同理：
I can't agree any more = I can't agree more.