The basic pronoun rules are that the pronoun has to match the original noun in number and case. For example, if you are talking about Sally, and she is the subject of the sentence, you must replace her with they. Look at the following example sentences for an explanation:

1. Sally eats an apple.
2. It eats an apple.
3. Her eats an apple.
4. They eats an apple.
5. She eats an apple.

In the first sentence, there are no pronouns being used. The subject of the sentence is Sally. The following sentences attempt to use pronouns to replace the subject of the sentence, Sally.Do you know which one is correct?
In the second sentence, Sally is replaced by “it.” Does this make sense? In general, it is not a good idea to replace a noun that is a person using “it” because “it” is gender neutral. If Sally was a horse, for example, then this sentence might make sense – but let’s assume Sally is a person. In this case, this second sentence is incorrect.
In the third sentence, Sally is replaced by “her.” Sally is the subject of the sentence, and therefore it has to be replaced by a nominative pronoun. “Her” is not a nominative pronoun - so this sentence does not make grammatical sense.  However, if someone said this sentence to you, you might understand what the speaker was saying. You would at least infer correctly that the person the speaker is talking about is female.
In the fourth sentence, we have used “they” to replace Sally. Does this example make sense? In fact, it does not. This is because Sally is a singular noun – we are only talking about one person. This means that we need to use a singular pronoun to replace the word Sally. “They” is a plural pronoun. Even though it is in fact nominative in case, it is not the right number, and therefore, this sentence is incorrect.
The final sentence, the fifth sentence, is the correct one: Sally is properly replaced by a singular, nominative pronoun.

Pronoun Usage in English

You might not be surprised that pronouns are some of the most commonly used words in the English language. You can replace almost any noun with a pronoun – this is the beauty of how effective these little words are. At the same time, if you do not follow the rules of pronoun usage in English carefully, you might confuse the reader or listener when you use them.