Maybe a synthesis of the following with the Hamsters long post may give the answer, is fronting involved too?
I also agree that there is sth more gramaticalised than a lexical phrase happening here, Juan.
Using "he", "she" or "it" helps to determine that "stare" is the infinitive and not the present. The only way to be sure is to use the verb to be.
This doesn't work exactly with this structure, but a similar structure would be:
He's a security guard so he doesn't have to do anything but be there.
I don't think "have to" alters anything here, although I suppose it's just possible that it might. This seems add greater evidence that "stare" is the infinitive.
I cannot accept, though, that "did" is an auxiliary, though, and this can be proved because it is possible to rewrite the sentence using the present perfect with "done" as the past participle:
He has done nothing all day but stare at the wall.
This website may be of help:
http://www.bbc.co.uk/worldservice/learn ... v226.shtml
You may want to look at it to see if you can get anything else out of it, but what appears to be the most relevent section reads:
There are one or two other structures where to-infinitive and the bare infinitive are both possible. Expressions with do or did, such as what I've done or all I did can follow either pattern.
I hate shopping so what I've done is (to) order a new computer over the Internet.
All I did was (to) suggest that she should lend him no more money. I didn't insist on it.
When two infinitive structures are connected by and, or or, except or but and than or as, it is normal practice to omit to in the second clause. Compare the following:
I would like you to tidy the house and (to) wash the dishes before I get home.
Would you prefer to have a snack now or (to) wait until later before we eat?
I could find nothing to do this afternoon, except read my book.
My son does nothing but watch TV when he gets home from school.
It's quicker to bike to the station rather than take the car.
I have to fix breakfast for everybody as well as take the children to school before I can leave for work.
The question is, why is it the bare infinitive? Why not, as Meg asked, the past simple, and why not the gerund? After all, we say:
She's interested in nothing but skiing; not
*She's interested in nothing but ski.
Maybe the verb after 'but' is determined by what goes before it. So: 'did...stare' and 'is interested in...skiing' and 'doesn't have to...be'.
But the present perfect example does not seem to accept this analysis.
The question is do we view the sentence:
"He did nothing but stare," as one or two clauses?
If it's two clauses joined by "but" then "did" is the main verb, no questions asked, if it's one clause, we could be looking at a type of complementation. Does anyone detect any mood/modality (overlay of meaning) here?
I can almost see this as one and a half clauses.
