NEITHER
Neither is used to show agreement with negative statements.
Neither + Auxiliary + Subject (pronoun)
The Auxiliary needs to agree with the verb tense in the original statement.
It is similar to using either at the end of a sentence, although Neither is more commonly used, especially in spoken English.
A: I don't understand Spanish.
B: Neither do I. I don't understand Spanish either.)
A: I cannot swim.
B: Neither can I. I can't swim either.)
Sometimes people respond Me Neither instead of Neither + Auxiliary + Subject though this is very informal spoken English.
SO
SO is used to show agreement with positive statements.
SO + Auxiliary + Subject (pronoun)
The Auxiliary needs to agree with the verb tense in the original statement.
It is similar to using TOO at the end of a sentence.
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