Agreeing and Disagreeing

Agreeing

Look at these sentences:

like music.
So do I.

live in California.
So do I.

This is the same as saying Me, too but in British English especially this form is more common.
Because the first sentence is in the present simple, the auxiliary verb is do:

So

Auxiliary verb

Subject

So

do

I

Look at this sentence:

worked yesterday.
So did I.

The verb is in the past simple so the auxiliary verb is did.

So

Auxiliary verb

Subject

So

did

I

Present Perfect:

have been to Japan.
So have I.

am watching television now.
So am I.

I‘m going to stay at home tomorrow.
So am I.

Note that the subject does not have to be “I”.

Examples:

have had lunch.
So has my mother.

They should stop smoking.
So should my parents.

When the sentence is negative and the other person is in the same situation, we use this form:

don’t live in Cuba.
Neither do I.

can’t speak Swahili.
Neither can I.

Note We can also use “nor”:

didn’t go out last night.
Nor did I.

Again, the subject doesn’t have to be I”:

I’m not going on holiday this year.
Neither is Phil.

She’d never been there before.
Neither had Sam.

Neither / nor

Auxiliary verb

Subject

Neither

do
did
have
should
can
would

I
You
They

Disagreeing

If the first person says a positive sentence and the second person wants to say a negative sentence or vice-versa, we can’t use so or neither because there is no agreement.

Look at these sentences:

I like tea.
don’t.

I haven’t been to Canada.
have.

The ideal response in this situation is to also use echo questions”.

Examples:

ate pizza last night.
Did you?

didn’t.

I‘ve been to China?
Have you?

haven’t.

I don’t like this music.
Don’t you? I do.