So … I
So + Auxiliary Verb + Subject
Use "so ... I" in a positive sense to show that we feel the same way as another person, or have performed the same action. Change the auxiliary verb related to the original statement. The form is usually used in the first person singular, however, other forms are also possible.
Examples:
He flew to Geneva last summer. - So did she.
I'd love to visit Poland some day. - So would I.
I'm meeting a colleague tomorrow. - So am I.
Neither … I
Neither + Auxiliary Verb + Subject
Use "neither ... I" in a negative sense to show that we feel the same way as another person, or have performed the same action. Change the auxiliary verb related to the original statement. The form is usually used in the first person singular, however, other forms are also possible.
Examples:
I haven't had a promotion for a long time. - Neither have I.
They weren't sure they had the resources to complete the job. - Neither were we.
She won't be able to attend the conference. - Neither will I.
Do you understand the rules? Test your knowledge with this So do I / Neither did I quiz.

