Pro-forms are words that are used to represent other words or phrases. A pronoun, if we retain this traditional term, properly represents a noun o r noun phrase or clause (though in this case it is hard to justify calling I, we and you pronouns, since they do not replace other nouns in the same way as he, she, it, and they), but adverbials and whole predicates can also be replaced by pro-forms. Quirk et al. (1972:§2.17) quote the following examples:
Mary is in London and John is there too.
Mary arrived on Tuesday and John arrived then too.
John searched the big room very carefully and the small one less so.
He often behaved prudently but he did not always behave thus/so.
She hoped that he would search the room carefully before her arrival but he didn't do so.
He didn't give her an apple. --Yes, he did.
They suspected that he had given her an apple and he had (done)
In the first four examples, there, then, so, and thus are pro-forms for the underlined adverbials, and could be called pro-adverbials (as distinguished from pronouns). In the last three examples, do so, did and had (done) (had done is more common in British English) replace the entire predicate of the preceding clause and could thus be called pro-predicates.
In the following sections errors involving various of these pro-forms are discussed.