I agree with the other posters' excellent answers. POSITIVE: I love English grammar. So do I. Me, too. I do, too. NEGATIVE: I don't like mushrooms. I don't, either. Me, neither. Neither do I. ***** Sometimes as a JOKE: Tom: I love ice cream. Mona: Me, too. Joe: Me, THREE!

When you want to express that you're feeling the same of think the same as someone else. For example: "I really like this song!" "So do I!" "I have a really bad headache." "So do I." It can be easily swapped with "me too" which sounds a little more natural/fluent. You wouldn't often use "so do I" when it come to negative expression. You should use "me neither". For example: "I don't like peas." "Me neither!" "I don't want to go to school tomorrow!" "Me neither." "So do I" is mostly used when expressing positive agreement. "Me neither" is used when expressing agreement with a negative statement.

If you want to say that you do the same as the other person without repeating the whole sentence. For example "I brush my teeth every day." "So do I!" "I like going to the beach." "So do I." For negative sentences you would use neither instead of so. "I don't like Mondays" "Neither do I"

it means something like "same with me". For example someone says "I don't really feel good right now." And if you feel something like that as well, you can say "So do I."