Easy to Say, Hard to Do
I know this is one of those rules that gets hammered into your head in grade school, but I came across a trickier example of this recently, so I decided to use it as a refresher.
The original sentence was (in essence):
The Heart Association and the Heart Foundation offer special training for both family and professional caregivers that teach techniques for dealing with the challenges of caring for someone with heart problems.
This is a tricky example because the main subject of the sentence (Association and Foundation) is plural, and the noun closest to the verb is also plural (caregivers)—so it's easy for the mind to "attach" the plural teach to either of those and completely miss that its real subject is training.
My Grammar Tip Trick for Subject/Verb Agreement
The way that I catch these kinds of mistakes is by focusing on the basic sentence structure as I read—I do a mental checklist as I'm reading a sentence. Once I identify a noun, I keep it in the front of my mind until I also identify its verb; then, I "see" them together to find if they agree.
With the example above, as I read it, I identified the nouns Association and Foundation and saw that the subject was plural. Then I came to the offer and it, too, was plural. So—clean slate! The next noun I came to was training, and I kept it at the front of my mind until I came to a verb—teach. "Training teach" obviously does not agree in number, so immediately I knew something's wrong and so I took the whole phrase and figured out how it should be:
The Heart Association and the Heart Foundation offer special training for both family and professional caregivers that teaches techniques for dealing with the challenges of caring for someone with heart problems.