Subject-verb agreement is an important aspect of English grammar that ensures that the subject and verb in a sentence agree in number and person. In other words, if the subject is singular, the verb should also be singular, and if the subject is plural, the verb should be plural as well. Here are the rules for subject-verb agreement:

1. Singular subjects:
When the subject of a sentence is singular, the verb should also be singular. For example:
- The cat is sleeping.
- She dances beautifully.

2. Plural subjects:
When the subject of a sentence is plural, the verb should also be plural. For example:
- The cats are playing.
- They dance beautifully.

3. Singular indefinite pronouns:
Indefinite pronouns like 'everyone,' 'someone,' 'anyone,' 'nobody,' etc., are considered singular, so the verb should also be singular. For example:
- Everyone is invited to the party.
- Nobody knows the answer.

4. Plural indefinite pronouns:
Indefinite pronouns like 'both,' 'few,' 'many,' 'several,' etc., are considered plural, so the verb should also be plural. For example:
- Both of them are talented musicians.
- Many people enjoy playing sports.

5. Collective nouns:
Collective nouns refer to a group of individuals as a whole. When a collective noun is treated as a single unit, the verb is singular. When it refers to the individuals within the group, the verb is plural. For example:
- The team is practicing. (referring to the team as a whole)
- The team are arguing among themselves. (referring to the individuals within the team)

6. Agreement with compound subjects:
When two or more subjects are joined by 'and,' the verb is plural. For example:
- Tom and Jerry are best friends.
- The dog and the cat are playing in the garden.

7. Agreement with compound subjects joined by 'or' or 'nor':
When two or more subjects are joined by 'or' or 'nor,' the verb agrees with the subject closest to it. For example:
- Neither the cat nor the dog is hungry.
- Either my parents or my sister is coming.

8. Agreement with phrases and clauses between the subject and verb:
Phrases or clauses between the subject and verb do not change the agreement. The verb should agree with the subject. For example:
- The book, along with the pen, is on the table.
- The girl who sings beautifully is my friend.

In conclusion, subject-verb agreement ensures that the subject and verb in a sentence agree in number and person. By following these rules, we can maintain grammatical accuracy in our writing and speaking.