So Do I / Neither Do I

So do I and neither do I are short responses that express agreement. So do I agrees with a positive statement; neither do I agrees with a negative one. They replace the need to repeat the full sentence.

Both structures use inverted word order: the auxiliary verb comes before the subject, not after it.

Agreeing with positive statements

The pattern is so + auxiliary + subject. The auxiliary must match the tense and verb of the original statement.

The subject can be any person, not only I. The auxiliary adjusts accordingly: so does he, so was Sarah, so had my brother.

Agreeing with negative statements

The pattern is neither + auxiliary + subject. Again, the auxiliary matches the original statement.

Neither always pairs with a positive-form auxiliary. The negation is already inside neither -- there is no need to add not or n't after the auxiliary.

Nor

Nor works exactly like neither and follows the same structure: nor + auxiliary + subject.

Nor is slightly more formal. In everyday conversation neither is far more common; in writing, both appear freely.

Choosing the right auxiliary

The auxiliary in the response must match what is in the original statement. If the original uses be, the response uses be. If the original uses a modal verb, the response repeats that modal. If there is no auxiliary at all (present simple or past simple with a regular verb), the response uses do / does / did.

Too, either, and the informal alternatives

English has several other ways to express the same agreement. The inverted so / neither form is the most recognizable in conversation, but these alternatives are equally correct.

Too and either in full sentences

For positive agreement, add too at the end. For negative agreement, add either at the end. Word order stays normal -- no inversion.

Me too / Me neither

The shortest possible responses. They work in any tense and with any verb because they skip the auxiliary entirely. Both are informal and perfectly natural in speech.

In more formal writing or speech, the inverted forms (so am I, neither have I) are preferred.

Common mistakes

Wrong word order (no inversion)

• "I like coffee." -- "So I do." --> "So do I."
• "She was late." -- "So I was." --> "So was I."

Double negative with neither

• "I don't swim." -- "Neither don't I." --> "Neither do I."
• "He can't cook." -- "Neither can't I." --> "Neither can I."

Wrong auxiliary

• "She is happy." -- "So do I." --> "So am I."
• "They went home." -- "So do I." --> "So did I."
• "We have finished." -- "So did I." --> "So have I."

So with negative / Neither with positive

• "I don't like rain." -- "So do I." --> "Neither do I."
• "I love summer." -- "Neither do I." --> "So do I."

Missing subject

• "I'm tired." -- "So am." --> "So am I."
• "He can't swim." -- "Neither can." --> "Neither can I."