TOO and EITHER
Look at the examples:
1. I like reading and she does, too.
2. He will come and we will, too.
3. I don’t like reading and she doesn’t, either.
4. He won’t come and we won’t, either.
From the examples above, we can see clearly that we use TOO for POSITIVE sentences while EITHER for NEGATIVE sentences to indicate SIMILARITY. The auxiliary before TOO and EITHER can be determined from the first auxiliary following the first subject (of course, we must also pay attention to the subject after the conjunction AND), those are “I” and “HE”.
Auxiliaries from examples number 1 and 3 are DO (derived from the verb 1 ‘LIKE’) and DON’T, thus the auxiliaries are DOES and DOESN’T (because the subject after the conjunction AND is SHE).
Auxiliaries from examples number 2 and 4 are WILL and WON’T, and since the auxiliaries are MODALS, you just simply put the same MODALS as the first one.
SO and NEITHER
Now, look at these examples:
1. I like reading and so does she.
2. He will come and so will we.
3. I don’t like reading and neither does she.
4. He won’t come and neither will we.
The use of SO and NEITHER is the same as TOO and EITHER, which is to indicate SIMILARITY. We use SO for positive sentences and NEITHER for negative sentences. The difference is the forms; when we use SO and NEITHER, we put the words SO and NEITHER after the conjunction AND. And the same as previous one, you can determine the auxiliary after SO and NEITHER from the first auxiliary following the first subject. HOWEVER!!! You CANNOT write negative form of auxiliary after the word NEITHER, since NEITHER has already carried a negative meaning.