Study materials
To agree with someone, Ukrainian says the equivalent of me too or me neither. English repeats the helping verb.
I love borscht. So do I.
I can't swim. Neither can I.
This article explains so and neither with a helping verb, too and either at the end, me too and me neither, and the formal as did her father.
So and neither
To say me too, use so, and to say me neither, use neither or nor. After them the helping verb comes before the subject, as in a question.
I like coffee. So do I.
I'm tired. So am I.
The first sentence has am, so am.
She hasn't called. Neither have I.
He can't come. Nor can we.
So follows a positive sentence, neither and nor a negative one. No not follows neither, because the word already negates. In conversation people often just say Me too or Me neither.
Too and either
The same can be said in normal word order. Then too comes at the end after a positive sentence, or either after a negative one.
I like coffee. I do too.
The same as So do I.
She hasn't called. I haven't either.
The same as Neither have I.
Too does not follow a negative: I'm not hungry either, not I'm not hungry too.
After as and than
In formal writing a helping verb can come before a noun subject after as or than. It is optional, and the usual order is also correct.
She studied law, as did her father.
The new model uses less fuel than does the old one.
Formal. Simpler is than the old one does.
With a pronoun the usual order stays: as he did, not as did he.
Common mistakes
The first mistake is not after neither.
I haven't seen it. Neither haven't I. → Neither have I.
Neither already negates.
The second mistake is too after a negative.
I'm not hungry. I'm not too. → I'm not either.
Either follows a negative.
The third mistake is an auxiliary that does not match the first part.
She has arrived. So do I. → She has arrived. So have I.
Has in the first part needs have in the second.
The first mistake shows after neither. The negative is already in it, so not is extra. The second shows at the end of a negative reply. Either stands there, not too. The third shows in the first sentence. The helping verb in the reply repeats it: has in the first sentence needs have, not do.