Why is the order of the words in "so do I" or "nor do I" different from the normal order?
With respect to information structure, this word order pattern seems to be equal to fronted adverbials, as in:
Into the room walked a man
The new information / focus follows the finite verb. Similarly, in "So do I", "so" refers to the proposition in the previous utterance, "do" is a dummy verb, and "I", the new information, comes last.
To agree with a positive statement:
We use so + auxiliary/modal verb + pronoun:
"I like tea without sugar.'
'So do I.'
To agree with a negative statement:
We use nor/neither + auxiliary/modal verb + pronoun:
"I don't like tea with sugar.'
'Nor do I.' or 'Neither do I.'
To disagree with a positive statement:
We use pronoun + auxiliary/modal verb + not (-n't):
"I like tea without sugar.'
To disagree with a negative statement:
We use pronoun + auxiliary/modal verb:
"I don't like tea with sugar.'
@itrekkie: I think you're on the right track.
Note that 'so' here has a different meaning from that it would have in the normal order ("I do so") - it means 'also', or 'as well' and it seems to me that it can have that meaning only when fronted. The fronting is clearly for emphasis, but that in itself is not enough to explain why the fronting is obligatory for that meaning. "As do I" and "Nor do I" don't have "as" or "nor" displaced, but they still have the inversion, and it seems to be crucial to this "as well" meaning. You can say "As I do", but without that meaning; and for some people "Nor I do" exists, but it means "and I do not", rather than "I do not either".
"Do" is syntactically a full verb, but semantically it's a place holder. But it's not crucial here: "So have I" and "So am I" are normal.
This isn't related to the issue of fronted "adverbials" as @Arne's accepted answer suggests. Those "adverbials" are usually prepositional phrases, as in the example given, which come after intransitive verbs. They don't require an auxiliary verb, and they don't work with transitive verbs. Consider:
- the plane flew over the fence --> over the fence flew the plane
- the boy flew the plane over the fence --> *over the fence flew the plane the boy
You can, however, do the "nor flip" with transitive verbs, and it does require an auxiliary verb, for example:
- nor will you see him again
You can do the same thing with a number of negative and limiting adverbs, and I think this is the pattern that nor belongs to.
- never did I say such a thing
- rarely is it on time
- seldom can they be trusted
Although it is mostly like a conjunction now, nor began life as an adverb according to the OED. The first example with so doesn't fit this pattern well because so is neither negative nor limiting. Nevertheless, this is the closest fit I can see.
What's going on here is the special role of auxiliary verbs:
- "do, "have", "be", "can", "shall", "may", "must", "might", "will"
and tenses of these. And in addition, the archaic or unusual
- "ought", "keep", "dare"
The main job of auxiliary verbs is to modify other verbs (when they stand alone, but that's a different meaning---like "to can" meaning to put stuff in cans), and they have the property that they are fronted in questions:
- have you gone?
- are you there?
- can you say this?
- shall we dance?
- aughtn't you stay?
- might we go?
- may I be excused?
- will we play?
- must we play?
Also the archaic or idiomatic sounding:
- Oughtn't you stay?
- Kept you going to the mall?
- Dare you believe this?
- Need I say more?
The verbs are also fronted in various constructions that modify the verb
- How quickly have you forgotten your promise.
- Only thus can you see this ship.
- In this manner shall we leave.
- Thus ought you dance.
- In this room will we play.
- All this must you believe, and more.
- Should I hold all this upon my shoulders, I will truly suffer.
- In all my days, may I never see your likes again.
- In the grandest fasion, so might we live.
The fronting property means that these verbs and modifiers which act to modify a second verb naturally occur before the subject, and then acquire their modifier meaning with the verb in ellipsis:
- I see a boat
- So do I.
exactly means "So do I see a boat". This is a grammatical sentence, just as
- I see a goat.
- So will I.
means "So will I see a goat". There is an ellipsis of the verb in this construction, leaving only the modifier.
- I see a moat.
- So should I.
Means "So should I see a moat." Again the ellipsis.
- I can see a float.
- So can I.
The motion is exactly the same as in the question. "Can I see a float?" and it doesn't work for verbs that are not auxiliaries, but where you introduce a "do". So
- I ate the bread
- So ate I.*
I ate the bread
If I'm hungry, so might I
I am allowed to go to the bathroom
So may I
I have to pee.
So must I.
These are all acting in the meaning of the verb ellipsis. The list above is the exhaustive collection of English things that appear before the subject.
Wikipedia gives the additional archaic or disfavored auxiliary verbs:
- dare ("I dare go to the cave")
- need ("I need eat this bread")
- keep ("I keep going".)
So that the construction is "I dare go to the cave"/"So dare I", "I need drink the medicine"/"So need I" "I keep working"/"So keep I". But these are no good. The problem with these constructions is that modern English basically requires a preposition: "I dare to go to the cave", "I need to eat this bread", "I keep on going" (in the case of keep, the on is skipped) and then it's like "I walk to the cave"/"So do I", not "So walk I". This is explained here, and there are a few phrases that survive from the modal days of "dare", "need", and "keep":
"how dare he!" (Wikipedia)
"Dare you question me?"
"Need I stay out?"
"Need I go on?"
So kept they going, past the mountains and the stream.
Explaining the surface behavior of this is easy, but describing why this is happening is a little harder. English generally has a SVO (Subject Object Verb) word order or syntax. The key concept here is that this word order is usually achieved through movement of words within the structure.
It's this structure I'm having trouble visualizing and this will require a lot more analysis to really figure out. If I had to bet, I'd say that first, this isn't an example of do-support insertion, that do here really is a verb, and that the so is its generated complement. Finally, instead of the regular movement to get our canonical word order, so is fronted (but why? again guessing, but to add emphasis to the agreement) to the position where I would normally move to, blocking its movement. do is tensed in its ordinary movement, and we end up with this strange word order.