I'm not sure which rule is the most important because all of them are. So here are all 20 of them.
1. Subjects and verbs must agree in number. This is the cornerstone rule that forms the background of the concept.
The dog growls when he is angry. The dogs growl when they are angry.
2. Don’t get confused by the words that come between the subject and verb; they do not affect agreement.
The dog, who is chewing on my jeans, is usually very good.
3. Prepositional phrases between the subject and verb usually do not affect agreement.
The colors of the rainbow are beautiful.
4. When sentences start with “there” or “here,” the subject will always be placed after the verb, so care needs to be taken to identify it correctly.
There is a problem with the balance sheet.Here are the papers you requested.
5. Subjects don't always come before verbs in questions. Make sure you accurately identify the subject before deciding on the proper verb form to use.
Does Lefty usually eat grass? Where are the pieces of this puzzle.
6. If two subjects are joined by and, they typically require a plural verb form.
The cow and the pig are jumping over the moon.
7. The verb is singular if the two subjects separated by and refer to the same person or thing.
Red beans and rice is my mom's favorite dish.
8. If the words each, every, or no come before the subject, the verb is singular.
No smoking and drinking is allowed.Every man and woman is required to check in.
9. If the subjects are both singular and are connected by the words or, nor, neither/nor, either/or, and not only/but also the verb is singular.
Jessica or Christian is to blame for the accident.
10. The only time when the object of the preposition factors into the decision of plural or singular verb forms is when noun and pronoun subjects like some, half, none, more, all, etc. Are followed by a prepositional phrase. In these sentences, the object of the preposition determines the form of the verb.
All of the chicken is gone.All of the chickens are gone.
11. The singular verb form is usually used for units of measurement.
Four quarts of oil was required to get the car running.
12. If the subjects are both plural and are connected by the words or, nor, neither/nor, either/or, and not only/but also, the verb is plural.
Dogs and cats are both available at the pound.
13. If one subject is singular and one plural and the words are connected by the words or, nor, neither/nor, either/or, and not only/but also, you use the verb form of the subject that is nearest the verb.
Do your sisters or your girlfriend want any pizza?
14. Indefinite pronouns typically take singular verbs.
Everybody wants to be loved.
15. * Except for the pronouns (few, many, several, both) that always take the plural form.
Few were left alive after the flood.
16. If two infinitives are separated by and they take the plural form of the verb.
To walk and to chew gum require great skill.
17. When gerunds are used as the subject of a sentence they take the singular verb form of the verb, but when they are linked by and they take the plural form.
Standing in the water was a bad idea.Swimming in the ocean and playing drums are my hobbies.
18. Collective nouns like herd, senate, class, crowd, etc. Usually take a singular verb form.
The herd is stampeding.
19. Titles of books, movies, novels, etc. Are treated as singular and take a singular verb.
The Burbs is a movie starring Tom Hanks.
20. Final Rule – Remember, only the subject affects the verb!
1. Subjects and verbs must agree in number. This is the cornerstone rule that forms the background of the concept.
The dog growls when he is angry. The dogs growl when they are angry.
2. Don’t get confused by the words that come between the subject and verb; they do not affect agreement.
The dog, who is chewing on my jeans, is usually very good.
3. Prepositional phrases between the subject and verb usually do not affect agreement.
The colors of the rainbow are beautiful.
4. When sentences start with “there” or “here,” the subject will always be placed after the verb, so care needs to be taken to identify it correctly.
There is a problem with the balance sheet.Here are the papers you requested.
5. Subjects don't always come before verbs in questions. Make sure you accurately identify the subject before deciding on the proper verb form to use.
Does Lefty usually eat grass? Where are the pieces of this puzzle.
6. If two subjects are joined by and, they typically require a plural verb form.
The cow and the pig are jumping over the moon.
7. The verb is singular if the two subjects separated by and refer to the same person or thing.
Red beans and rice is my mom's favorite dish.
8. If the words each, every, or no come before the subject, the verb is singular.
No smoking and drinking is allowed.Every man and woman is required to check in.
9. If the subjects are both singular and are connected by the words or, nor, neither/nor, either/or, and not only/but also the verb is singular.
Jessica or Christian is to blame for the accident.
10. The only time when the object of the preposition factors into the decision of plural or singular verb forms is when noun and pronoun subjects like some, half, none, more, all, etc. Are followed by a prepositional phrase. In these sentences, the object of the preposition determines the form of the verb.
All of the chicken is gone.All of the chickens are gone.
11. The singular verb form is usually used for units of measurement.
Four quarts of oil was required to get the car running.
12. If the subjects are both plural and are connected by the words or, nor, neither/nor, either/or, and not only/but also, the verb is plural.
Dogs and cats are both available at the pound.
13. If one subject is singular and one plural and the words are connected by the words or, nor, neither/nor, either/or, and not only/but also, you use the verb form of the subject that is nearest the verb.
Do your sisters or your girlfriend want any pizza?
14. Indefinite pronouns typically take singular verbs.
Everybody wants to be loved.
15. * Except for the pronouns (few, many, several, both) that always take the plural form.
Few were left alive after the flood.
16. If two infinitives are separated by and they take the plural form of the verb.
To walk and to chew gum require great skill.
17. When gerunds are used as the subject of a sentence they take the singular verb form of the verb, but when they are linked by and they take the plural form.
Standing in the water was a bad idea.Swimming in the ocean and playing drums are my hobbies.
18. Collective nouns like herd, senate, class, crowd, etc. Usually take a singular verb form.
The herd is stampeding.
19. Titles of books, movies, novels, etc. Are treated as singular and take a singular verb.
The Burbs is a movie starring Tom Hanks.
20. Final Rule – Remember, only the subject affects the verb!
