Ellipsis and substitution

Fluent English avoids repeating itself. An auxiliary can stand for a whole verb phrase: she can swim and so can I, I haven't finished, but Tom has. So and not replace a clause: I think so, I hope not.

One and ones replace a countable noun: I don't like this shirt; I prefer the blue one. Do so replaces an action in formal writing: he promised to call, and he did so.

After so, neither and nor, the auxiliary comes before the subject: so do I, neither did we. Match the tense and the auxiliary of the first clause.