from Hunter College:
Most people think of English as having three main verb tenses: Past, Present, and Future, and grammar books identify a total of twelve tenses, including a Perfect, Progressive, and Perfect- Progressive tense to go along with each of the three main tenses. But most linguists agree that English actually has a two-tense system – Present and Past. Looking at the verb system of English this way is both simpler and more suitable to the actual structure of the language. English sentences are framed around a time frame of either THEN (the Past) for actions that are completed or NOW (the Present) for actions that are not completed. All the other verbs, including perfect, progressive, and future forms are expressed in relationship to one of these two time frames; they refer to BEFORE, DURING, and AFTER the main time frame, and they all contain either a past tense or a present tense form.
Read the rest: The Two-Tense Verb System Text