Pro-Form in Grammar

Pro-form is a word or phrase that can take the place of another word (or word group) in a sentence. The process of substituting pro-forms for other words is called proformation.

In English, the most common pro-forms are pronouns, but other words (such as here, there, so, not, and do) can also function as pro-forms. 

The pro-form is the referring word in a sentence; the word or word group that's referred to is the antecedent.

Examples and Observations:

• "My grandmother started walking five miles a day when she was sixty. She's 97 now, and we don't know where the hell she is."  (American comedian Ellen DeGeneres)
• "Our father ...came back in the morning and told us he had found lodgings, and so we went there. They were east of the harbour, off Lot Street, at the back of a house which had seen better days." (Margaret Atwood, Alias Grace. McClelland & Stewart, 1996)
• "One day in English class I passed Bill Hilgendorff a note. 'I love you,' the note said. He folded it up and looked straight ahead. Then I whispered to him that he could live his whole life long and no one would ever love him as I did. I thought this was an amazing and daring and irresistible thing to do." (Tereze Glück, May You Live in Interesting Times. University of Iowa Press, 1995)
• "We had offers to play in Hong Kong, and I always wanted to go there, but I wouldn't agree to do it because it wasn't going to add any more profit to the tour." (Johnny Ramone, Commando: The Autobiography of Johnny Ramone. Abrams, 2012)
• "When the tzar was seated, everyone else sat, and so did we." (L. E. Modesitt, Jr., Ghost of the White Nights. Tor Books, 2001)
• "Boldly, Stein leaps from her short history of open source to the future of Canadian federalism. One might have expected her to develop her argument in the direction of scenario IV, but unfortunately she did not." (Ruth Hubbard and Gilles Paquet, The Black Hole of Public Administration. University of Ottawa Press, 2010)
• "I'm proud of the many ways you're growing, and I hope you are, too." (Fred Rogers, Dear Mr. Rogers. Penguin, 1996) Rogers, Does It Ever Rain in Your Neighborhood?: Letters to Mr.
• "People cheerfully mixed the spiritual and the mundane, and I did as well." (Gwendolyn M. Parker, Trespassing: My Sojourn in the Halls of Privilege. Houghton Mifflin, 1997)

So and Not as Pro-Forms 

"Sometimes pro-forms represent less clearly identifiable constituents:

(6) He may decide to join us next week, but I don't think so.
(7) Speaker A: Will he join us next week?
Speaker B: I hope not.

In (6) the adverb so represents the preceding clause but with an appropriate change of operator: . . . but I don't think he will join us next week. In (7), the adverb not represents the whole of the preceding clause but changes it into a negative statement: . . . I hope he will not join us next week." (Carl Bache, Mastering English: An Advanced Grammar for Non-Native and Native Speakers. Walter de Gruyter, 1997)

Do as a Pro-Form

"Do is used as a pro-form when the predicate itself and all the complements which follow it are elided (Jack hurt himself fetching water, and Jill did, too). If another auxiliary is present, the pro-form do is less common (Has Jack hurt himself? Yes, he has; also, Yes, he has done . . .). Note that the pro-form do is not the same lexeme as the auxiliary do; the latter has only the forms do, does, did while the pro-form has these as well as done and doing." (Stephan Gramley and Kurt-Michael Pätzold, A Survey of Modern English, 2nd ed. Routledge, 2004)