The main difference between "tense" and "tenses" is that "tense" refers to the grammatical form of a verb that indicates the time when an action occurred (past, present, or future), while "tenses" is the plural form of "tense" and refers to the different types of tenses in a language. Tenses in English include present, past, and future, as well as variations like present perfect, past perfect, and future perfect.