Pronoun agreement is also known as antecedent agreement. It forms one of the bases of proper English sentence construction. Ignoring the rule of pronoun agreement causes confusion about meaning of the sentence.
A pronoun is defined as a word used to refer to a noun in a long sentence or paragraph. In the sentence, "I went to Harry’s house but he was not in," Harry is the noun and the word used to substitute for Harry in the latter part of sentence is the pronoun. Thus, the word "his" is the pronoun.
Pronoun Number Agreement
Consider the following incorrect sentence. "I went to Harry’s house but they were not in." What is wrong with this sentence? The number of nouns does not match the pronoun number. This means there is a disagreement between noun and pronoun number. The noun, Harry, refers to one person and is singular. But the pronoun, "they"refers to more than one and is plural.
Pronoun Gender Agreement
Consider the following incorrect two sentences. "I borrowed Harry’s book yesterday. I will be returning her book back today." Does the noun match the pronoun gender? Harry is a masculine gendered noun while the pronoun refers to female gender. This is a disagreement of noun-pronoun gender.
Pronoun agreement rules apply rigidly to written English but not to conversational English. To write "someone lent me their book" is improper English as there is number and gender noun-pronoun disagreement. Someone is singular but the pronoun used, "they," is plural.
Speaking the same sentence in a conversation is proper and acceptable. Improper pronoun agreement is accepted in common English conversation jargon. When you say, someone lent me their book, previous knowledge and schemas about the background color your understanding of the sentence. You comprehend the exact meaning of the sentence despite the pronoun disagreement.