We agree with positive statements using so-too:
I have a red car.
(Sara) So does Sara
But with the negative statements we use neither-either:
I don't have a brother.
(Jhon) Neither does Jhon
So I can say that the most important thing before giving your agreement is to know whether the sentence is positive or negative and then to agree as I did in the examples above.
But these sentences let me think many times:
I have nothing to do.
(I) so do I
or
Neither do I
So the question is:
Should I consider the last sentence positive or negative?
Look at this sentence too:
The boy is impolite (It means that the boy is not polite)
(girl) so is the girl
or
Neither is the girl
I mean if there's a prefix (which changes the word to the opposite), then the sentence will be negative in meaning so either or neither is used?
>
- you can see this in that meta post I linked to in the original comment. However, your original question as nearly unreadable, as markdown doesn't insert linebreaks if there isn't an extra empty line or two spaces following the previous paragraph.