Plan: pronoun

Pronoun versus pro-form
Pronoun is a category of words. A pro-form is a type of function word or expression that stands in for (expresses the same content as) another word, phrase, clause or sentence where the meaning is recoverable from the context.^[4] In English, pronouns mostly function as pro-forms, but there are pronouns that are not pro-forms and pro-forms that are not pronouns.^{[5][p. 239]}

Pronouns versus Pro-forms
	Example	Pronoun	Pro-form
1	It is a good idea.	✓	✓
2	I know the people who work there.	✓	✓
3	Who works there?	✓
4	It is raining.	✓
5	I asked her to help, and she did so right away.		✓
6	JJ and Petra helped, but the others didn't.		✓

Examples [1 & 2] are pronouns and pro-forms. In [1], the pronoun it "stands in" for whatever was mentioned and is a good idea. In [2], the relative pronoun who stands in for "the people".
Examples [3 & 4] are pronouns but not pro-forms. In [3], the interrogative pronoun who does not stand in for anything. Similarly, in [4], it is a dummy pronoun, one that does not stand in for anything. No other word can function there with the same meaning; we do not say "the sky is raining" or "the weather is raining".
Finally, in [5 & 6], there are pro-forms that are not pronouns. In [5], did so is a verb phrase that stands in for "helped", inflected from to help stated earlier in the sentence. Similarly, in [6], others is a common noun, not a pronoun, but the others probably stands in for the names of other people involved (e.g., Sho, Alana, and Ali), all proper nouns.
Grammar
Pronouns (antōnymía) are listed as one of eight parts of speech in The Art of Grammar, a treatise on Greek grammar attributed to Dionysius Thrax and dating from the 2nd century BC. The pronoun is described there as "a part of speech substitutable for a noun and marked for a person." Pronouns continued to be regarded as a part of speech in Latin grammar (the Latin term being pronomen, from which the English name – through Middle French – ultimately derives), and thus in the European tradition generally.
Because of the many different syntactic roles that they play, pronouns are less likely to be a single word class in more modern approaches to grammar.^[6]