How to agree or disagree in English

Фразы типа "So do I" и "Neither do I" употребляются как отклики, показывающие ваше похожее отношение, действие или состояние к тому, что сказал ваш собеседник. Вспомогательный глагол в вашем ответе такой же, как во фразе, на которую вы откликаетесь.

So + Auxiliary Verb + Subject

I like oranges. – So do I.

I visited India last year. – So did I.

I was very disappointed. – I was too. / So was I.

Neither + Auxiliary Verb + Subject

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.