Your description of pronouns is only partially correct. A pronoun replaces a *noun phrase*, not a noun. In other words, it replaces the noun and all of its surrounding modifiers. Take your first example, “The farmer plows his field … ” ‘He’ does not replace ‘farmer’ only, but also ‘the.’ If it replaced only ‘the farmer’, we would get the ungrammatical sentence “The he plows his field … ” Similarly, if we want to replace ‘his field’ with a pronoun, the pronoun ‘it’ would “erase” both ‘his’ and ‘field,’ not only ‘field’: “The farmer plows it,” not “The farmer plows his it.” Regardless of how long a noun phrase is, a personal pronoun must “erase” the whole thing. Take, for example, the sentence “The women vying for Olympic Gold in the one-hundred -meter sprint represent five different countries.” To pronominalize the subject, we would have to erase the entire participial modifier along with the article and the head noun: “They represent five different countries.” If we use ‘they’ to replace only ‘women,’ we get the absurdity “The they vying for Olympic Gold in the one-hundred-meter sprint represent five different countries.” The only pronoun that truly replaces only the head (or main) noun in a noun phrase is ‘one’ or ‘ones’: “The ones vying for Olympic Gold in the one-hundred-meter sprint represent five different countries.” I hope you will amend your grammar text to reflect this more-accurate description of pronoun function in English.