"So" and "too" are used in positive sentences to show agreement.
Positive Sentence, Too:
SO + AUXILIARY + SUBJECT
Examples:
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Person A: I love chocolate.
Person B: I love chocolate, too.
So do I.
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Person A: Sarah is a talented singer.
Person B: I can sing well, too.
So can I.
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Person A: They have visited Paris.
Person B: I have been to Paris, too.
So have I.
"Either" and "neither" are used in negative sentences to show agreement. (NEITHER = NOT EITHER)
Negative Sentence, Either:
NEITHER + AUXILIARY + SUBJECT
Examples:
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Person A: I don't love football.
Person B: I don't love football, either.
Neither do I.
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Person A: Jack can't swim.
Person B: I can't swim, either.
Neither can I.
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Person A: Hana and Jim haven't done their tasks.
Person B: I haven't done my tasks, either.
Neither have I.