So, Too, Either, Neither

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.

Person A	Person B
I am happy.	So am I.	= I am happy too.
I'm going to Brazil in the summer.	So am I.	= I am going to Brazil too.
You look nice today.	So do you.	= you look nice too.
Stephanie has a new boyfriend.	So does Mary.	= Mary has a new one too.
We went to the concert last night.	So did I.	= I went to the concert too.
I would love a coffee right now.	So would I.	= I would love a coffee too.
He will win a prize.	So will I.	= I will win one too.
They have finished their homework.	So have I.	= I have finished too.
I can speak two languages.	So can I.	= I can speak two too.
He should study more.	So should I.	= I should study more too.
We could see the mountains.	So could we.	= We could see them too.
My brother had eaten too much.	So had I.	= I had eaten too much too.

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.

Person A	Person B
I am not hungry.	Neither am I.	= I'm not hungry either
I'm not going to quit.	Neither am I.	= I'm not going to quit either
They don't speak French.	Neither do I.	= I don't speak French either.
Stephanie doesn't eat meat.	Neither does Mary.	= Mary doesn't eat meat either.
Mary didn't go to the party.	Neither did I.	= I didn't go either.
I wouldn't like to do his job.	Neither would I.	= I wouldn't like to do it either.
He won't stop talking.	Neither will you.	= You won't stop either.
You haven't finished your meal.	Neither have you.	= You haven't finished either.
Mike can't reach the top shelf.	Neither can I.	= I can't reach it either.
You shouldn't talk in the movie.	Neither should you.	= You shouldn't talk either.
We couldn't hear him.	Neither could we.	= We couldn't hear him either.
I hadn't seen her before.	Neither had I.	= I hadn't seen her before either.