It is usually pretty easy to match the verb with the subject in English. Only in the present tense does the verb have more than one form. And except for one verb, only the third person singular is different. Besides, the third person singular present tense always ends in an s. We understand this most of the time.
Verb: To speak
I, you, we, they speak
he, she, it speaks
Verb: To do
I, you, we, they do
he, she, it does
Verb: To be (the only exception)
you, we, they are
he, she, it is
The verb to be is also the only verb with more than one form in the past tense. See also the subjunctive mood.
Verb: To be, past
I, he, she, it was
you, we, they were
Normally, none of this is a problem. However, there are a few cases that confuse writers and speakers.
Separated Subjects and Verbs
A phrase or clause often separates the subject and the verb. The verb must still agree with the subject.
Incorrect: The climate in both places are mild.
Correct: The climate in both places is mild.
(Climate is the subject, not places. It takes the verb is.)
Keep track of the subject, especially when there is a singular pronoun or collective noun for the subject and a plural element in the phrase that separates the subject and verb.
Collective noun: A group of senators was calling for an investigation.
Singular pronoun: One of the many galaxies was proven to be near a black hole.
Two or more singular subjects joined by or or nor take a singular verb.
Correct: Neither John nor Mary knows what happened.
Two or more plural subjects joined by any conjunction (including and, or, but, or nor) take a plural verb.
Correct: Both men and women are allowed to enter.
If one or more singular subject is joined to one or more plural subject by or or nor, the verb agrees with the subject closest to the verb.
Incorrect: Neither Mary nor her brothers knows what happened.
(Brothers is closer to the verb and is plural; the verb should agree with brothers).
Correct: Neither Mary nor her brothers know what happened.
Correct: Neither her brothers nor Mary knows what happened.
A compound subject whose parts are joined by and normally takes a plural verb.
Correct: Joe and his brother know what happened.
A compound subject whose parts are joined by and takes a singular verb in two special instances.
1. When the parts of the subject combine to form a single item.
Correct: One and one equals two.
Correct: Cookies and cream is my favorite flavor.
2. When the compound subject is modified by the words each or every.
Correct: Every boy and girl has to participate.
See also British vs. American Grammar, The Verb To Be, and Indefinite Pronouns.