Subject verb agreement

Subject-verb agreement means the verb form must match whether the subject is singular or plural.
One thing takes a singular verb: "The scientist studies bats."
More than one takes a plural verb: "The scientists study bats."

The hard part is finding the true subject.
Words often sit between the subject and its verb, and they can fool you.
In "The box of chocolates is on the table", the subject is box (singular), not "chocolates".
Cross out the in-between phrase "of chocolates" and the match becomes clear.

Watch for a couple of common traps.
After the word "that", the verb usually refers to the noun just before it: in "gases that trap heat", the verb "trap" matches gases (plural).
A phrase set off by commas can also be ignored: in "The flutes, made of bamboo, are ancient", the subject is flutes, so the verb is "are".

So the method is always the same.
Find the noun that is actually doing the verb, decide if it is one or many, and pick the matching verb form.
Do not let the nearest word decide for you.

Choose the correct verb: "The bouquet of red roses ______ beautiful."
(A) is
(B) are
The subject is bouquet, which is singular; "of red roses" is just an in-between phrase.
A singular subject needs a singular verb, so (A) "is" is correct.

Choose the correct verb: "The players on the winning team ______ hard every day."
(A) trains
(B) train
The subject is players (plural); "on the winning team" is an in-between phrase.
A plural subject needs a plural verb, so (B) "train" is correct.

Choose the correct verb: "Carbon dioxide is one of the gases that ______ the sun's heat."
(A) traps
(B) trap
After "that", the verb refers to the noun just before it, which is gases (plural).
A plural subject needs a plural verb, so (B) "trap" is correct.