Subject-verb agreement means that a verb takes the right form to match its subject. For example, a singular first-person subject, "I," gets paired with the first-person singular verb "am" instead of another form like "is" or "are." Even for people who've never heard this grammar terminology, alarm bells go off immediately when they hear something like "I is," "I are" or "I be."
The only time subject-verb agreement is difficult is when a complicated sentence causes you to lose track of the subject: "The North American predator bird, a group whose members include hawks and falcons and eagles, are found in different regions of the country."
That sentence contains an error — "are" should be "is" to match the singular "bird." Mistakes like this happen because a writer gets thrown by all the plurals that came after "bird" and not because he actually believes it's right to say "The bird are found."
But while subject-verb agreement is pretty easy, subject-object agreement can be impossible. Literally.
Consider these sentences: "All the people in the audience raised their hand." "Homeowners should check to make sure that the insurance policy they choose has a low deductible." "In the United States, very few teenagers own a motorcycle."
Clearly, people don't share a single hand, homeowners don't share a single policy with a single deductible, and teenagers don't share a single motorcycle. But what happens when we try to make these more logical?
"All the people in the audience raised their hands" sounds as though every person there raised two hands into the air.