So do I. Neither do I.

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So do I. Neither do I.

Phrases like "So do I" and "Neither do I" are used as responses showing your similar attitude, action, or state in comparison with what your conversation partner has said.
The auxiliary verb in your response is the same as in the phrase to which you are responding. If there are two auxiliary verbs in the phrase to which you are responding, use the first auxiliary verb in your response.
If there is no auxiliary verb in the phrase to which you are responding (i.e., the main verb is in the Simple Present or the Simple Past), use the auxiliary verb DO (does, did) in your response.

Responses to affirmative phrases

I am a student. – So am I.
I like oranges. – So do I.
I feel terrible. – So do I.
I was there yesterday. – So was I.
I visited India last year. – So did I.
I will see him tomorrow. – So will I.
I have been waiting for a long time. – So have I.

Responses like "I do too" may be used instead of responses like "So do I", with the same meaning.

I'm bored. – I am too. / So am I.
I like it. – I do too. / So do I.
I was disappointed. – I was too. / So was I.

Responses to negative phrases

I am not hungry. – Neither am I.
I don't know him. – Neither do I.
I didn't call them. – Neither did I.
I wasn't laughing. – Neither was I.
I won't go there. – Neither will I.

Responses like "I don't either" may be used instead of responses like "Neither do I", with the same meaning.

I don't like it. – I don't either. / Neither do I.
I didn't find her. – I didn't either. / Neither did I.
I wasn't invited. – I wasn't either. / Neither was I.

Note: Me too. Me neither.

Informal responses "Me too" and "Me neither" are sometimes used in colloquial speech in informal situations. (Language learners should avoid such use in their speech and writing.)

I am hungry. – Me too.
I wasn't sure. – Me neither.

The verb HAVE in responses

If the phrase to which you are responding has the auxiliary verb HAVE, use the verb HAVE in your response. If the verb HAVE is used as a main verb (meaning: to own, to possess), use the verb DO (does, did) in your response.

I have finished my work. – So have I.
I haven't seen this film yet. – Neither have I.
I have been working very hard lately. – So have I.
I have a question. – So do I.
I have a car. – So do I.
I have no time. – Neither do I.
I don't have any money. – Neither do I.
I had a lot of work yesterday. – So did I.

Expressing similarity with other people or objects
First person phrases and responses are the most common, but various other combinations are also possible.

She is on vacation now. – So are we. / We are too.
John can speak Italian. – So can Tom. So can Tina.
Maria works in this building. – So does Anton. So do we.
This house is very expensive. – So is that house. So are those houses.
She didn't ask him about it. – Neither did you. / You didn't either.
You shouldn't stay here. – Neither should Alex.
My task wasn't very difficult. – Neither was mine.
My pen doesn't write. – Neither does mine. / Mine doesn't either.

Note: Use of "nor"
If several people or things are mentioned in your negative response, use "neither" with the first and "nor" with the second, third, etc.

She didn't ask him about it. – Neither did you. Nor did his brother.
I never saw him after that. – Neither did I, nor did Mike.
You shouldn't stay here.