Which verb do I have to use in the following sentence:
The majority of the class is/are having the same problem.
“Majority” is one of those words that can be either singular or plural. Common sense works pretty well in deciding which. If you mean the word to describe a collection of individuals, then the word should be treated as plural: “The majority of e-mail users are upset about the increase in spam.” If the word is used to describe a collective group, then consider it singular: “A 90% majority is opposed to scheduling the next meeting at 6:00 A.M.”