To agree with a positive statement:
We use so + auxiliary/modal verb + pronoun:"I like tea without sugar.'
'So do I.'
To agree with a negative statement:
We use nor/neither + auxiliary/modal verb + pronoun:"I don't like tea with sugar.'
'Nor do I.' or 'Neither do I.'
To disagree with a positive statement:
We use pronoun + auxiliary/modal verb + not (-n't):"I like tea without sugar.'
'I don't.'
To disagree with a negative statement:
We use pronoun + auxiliary/modal verb:"I don't like tea with sugar.'
'I do.