Everything about subject-verb agreement follows this basic rule:
The finite verb agrees with the subject in person and number.
Four concepts are involved here...
finite verb
subject
person
number
The agreement between the subject and the verb is not as difficult as it seems. This page will guide you through the fundamentals of the topic.
The Subject-Verb Agreement Story
In our subject-verb agreement story, we have two characters and two character traits.
The two characters are...
The subject
The finite verb
For a general understanding of these two characters, click here. If you want a more intimate understanding of them, click these links: subject and finite verb.
A man sits at the counter. Two men sit at the counter.
a man is singular; so we have the singular verb sits.
two men is plural; so we have the plural verb sit.
Good! Singular and plural takes care of the number; but, what about the person part of the rule?
So let's complete the basic story.
Here's the bit that remains.
Look at these examples:
I am sitting at the computer.
He is sitting at the computer.
The subjects 'I' and 'He' are both singular. Why then are the verbs different?
It's because they belong to different grammatical persons.
The subject 'I' is in the first person; so the verb is 'am sitting.'
The subject 'He' is in the third person; so we have 'is sitting' - a different form of the verb.
So let's repeat the fundamental lesson
The finite verb agrees with the subject in person and number.
Not "number" only, but "person and number."
Subject-Verb Agreement for Simple Subjects
We now know what finite verbs do.
A finite verb has a good look at the subject (from top to bottom)! In that one look, it determines the person and number of the subject, and then it responds appropriately with its own sweet form!
Agreement in the Present Tense
Let's take an action verb first...
"to sing"
singular
plural
First Person
I sing
We sing
Second Person
You sing
You sing
Third Person
He/She/It/ any singular subject sings
They/ any plural subject sing
Note that the third person singular subject has sings.
For the verb 'to be', you have...
I am, we are, you are, he/she/it/singular is, they/plurals are.
Note the difference in the verb in two places: first person singular and third person singular.
For the verbs 'to have' and 'to do'...
'to have' - I have, we have, you have, he/she/it has, they have.
'to do' - I do, we do, you do, he/she/it does, they do.
Agreement in the Past Tense
It is necessary to remember only the different forms of the verb to be; the other verbs have the same form throughout.
'to be' - I was, we were, you were, he/she/it was, they were.
'to sing' - I sang, we sang, you sang, he/she/it sang, they sang.
Will, Shall, Can, Could, and Other Modals
When the first word of a finite verb phrase is a modal (e.g. will or shall, as in "future tense" verbs), we need not worry about subject-verb agreement.
Modals don't change form. Also, the form of the next verb after a modal is always the base form (i.e. the bare infinitive).
Examples: will sing, will be, will have, can sing, can be, can have, etc.
Agreement with Compound Subjects
Subjects which have more than one element in them are called compound subjects. Examples of compound subjects are: he and she, he or she, you or they.
The problem will be to determine the number or person of the subject so that the rule can be applied.
The rule itself remains unchanged:
The finite verb agrees with the subject in person and number.
I suggest you do a grammar exercise on what we have learnt on this page.