A subject and verb must agree in number (A singular noun needs and singular verb, and a plural noun needs a plural verb).
Example: Bob dates Staci. Vegans do not eat meat.
Do not be confused by the separation of noun from verb; in most cases, only the noun matters.
Example: The pair of pants is one the bed ("pair" is singular as is "is").
Example: The teacher as well as the students is present ("teacher" is all that affects the verb).
Do not be confused by words following the verb.
Example: The main crop is carrots (While "carrots" is plural, "crop" is the singular subject).
Be careful with "or" and "nor." The noun closest to the verb affects the verb.
Example: The mother or her children are coming to register.
Example: The children or their mother is coming to register.
Be wary of indefinite pronouns.
Anyone, everyone, each, no one, someone, something, and none (when meaning not one) are singular.
Some, all, any, or none can be singular or plural, depending on what they refer to.
Example: Everyone is in his or her home.
Example: Something has to be done.
Example: Each of the students has a test on Tuesday.
Example: Some of the flowers are dead. Some of the work is done.
Example: All of the smoke has cleared. All of the steroids are illegal.
Collective nouns can be treated as singular or plural, depending on the context.
Example: The class has decided to do the extra credit (one unit).
Example: The class go their separate ways after school (individuals).
Pronouns and their antecedents must also agree.
Example: Everyone sleeps in his or her own bed. People sleep in their beds.
Example: A student must learn to correct his or her mistakes. Students must learn to correct their mistakes.
Example: Either the traps or the mouse will have its day. Either the mouse or the traps will have their day.
Collective pronouns agree in the same manner as do collective nouns.
Example: The group give its opinion (one unit's opinion).
Example: The group gave their opinions (individuals' opinions).