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A pro-form is a word that can be used as a replacement to avoid redundancy without removing meaning from a sentence.
Its best known example is the pronoun.
With some mental flexibility, you can also see pro-verbs and even pro-sentences - consider "Yes.".
For adjectives and adverbs it is harder to demonstrate.
In "Can I kick it? Yes, you can.", you can say that 'can' is a replacement of the previous sentence. Or that the omission of a subject implies that reference.
The second sentence is understood as "Yes, you can kick it."
It itself is also a pro-form, specifically a pronoun, a reference to whatever it is we are supposed to be kicking.
A pro-sentence refers to one or a few words referring to an entire sentence or a large part of it.
In english, an extreme case is the answer "Yes." It's not a valid independent sentence, but an an answer to the first sentence before, meaning something like "Yes, you can kick it."
This could be seen as a forceful stretch preferred by those for whom analysis depends on sentences having all basic parts. In discourse analysis - which is probably the most useful for uttered dialogue anyhow - this is not necessarily necessary.