This heading sounds extremely weird, but it points to an extremely important part of sentence correction.
This is based on subject pronoun agreement. In a lot of exams, this is an extremely important subject, and a lot of questions come from sentence correction. Thus it is very important to know it thoroughly.
Let us look at the important terms:
Subject of the sentence: The subject is what the sentence is about. It is usually the name of a person or thing. It is almost always a noun or a pronoun. e.g. John, Mary, Alice, Peter, India, country, bird, flower, I, you, he, she, it, we, they etc.
Verb - The verb indicates an action. e.g. work, sing, dance, play, write, run etc.
action words e.g. work, swim, dance, play etc.
Object - the rest remaining part of the sentence is the object.
Prepositions - Prepositions are the words which tell the position of nouns and pronouns. e.g. to, from, in, at etc.
Let us look at some examples:
Example 1: The teacher began her lecture.
in this case the subject of the sentence is the teacher, and since the teacher i us singular, hence the pronoun to be used with it will also be singular.
Example 2: The teachers began their lecture.
in this case the subject, the teachers is plural and hence the pronoun will also be plural.
Example 3: Uncle Henry travelled oversees to buy vases for her company.
in this case the gender of the subject, changes the pronoun and hence this sentence is incorrect.
The correct sentence would be: Uncle Henry travelled oversees to buy vases for his company.