Even native English speakers will have trouble identifying subjects (and verbs) in sentences that deviate from the most basic, such as “the dog barks.” Here there are only an article (the), a subject/noun (dog) and a verb (barks). Easy enough.
However, look at this sentence and its optional verb forms and tell me the answer:
“A variety of options concerning repair of the train cars is/are on the table.”
Which verb is it, singular or plural?
To answer that, you have to determine the correct subject. Is the subject repair, options, train cars or what? Actually, it’s variety. Being a singular noun, variety takes the singular verb form is.
In other words, “A variety…is on the table.” The rest of the sentence just fills in the meaning without altering the subject but complementing or explaining it.