Tense versus aspect
Some languages do not have verb tenses. The English language has only one tense: the simple past tense, also known as the preterite tense, which signals that an action was completed at a specific time in the past. ESL teachers, like me once upon a time, confuse students by saying that English verb tenses refer to the past, the present or the future, but they don't really. Once you start teaching verbs in detail you realize that we use modal auxiliaries like "will" and "going to" to refer to the future. What we traditionally call "the present tense" refers to the present, past and future, as in the examples "I live in Canada" or "The population of Sao Paulo is 10 million." The more difficult and significant distinction among English verbs are aspects like habitual (I study), continuous (I am studying), perfect (I have studied) and perfect continuous (I have been studying), which usually get taught as being different tenses.
Twelve tenses or four aspects?
The truth is that when I was teaching issues like verb tense I, like everyone else, always instructed my students that there were twelve different “tenses” in English. In hindsight I recognize that by identifying the various forms of verb as referring to the past, the present or the future, I was mislabelling what the various forms indicated and necessarily misleading and confusing my students. The crucial concept is not “tense” but “aspect” and most grammar books destined to instruct students learning English don’t even mention the concept of “aspect.”
Tenses do not correspond to the time frames which give them their names
Describing verbs as being “past,” present” or “future” is (with the exception of the past) meaningless and misleading. The fact that English verbs can be “simple” (or habitual/repeated), “continuous” (or progressive, the French “imperfect” is sensible), “perfect” and “perfect continuous” is much more significant and meaningful. Teaching “aspect” is a much more promising approach for getting the variations across to students than the self contradictory tradition of referring to every form of the verb as a “tense.” The only way to prove my point is to consider each of the so-called “verb tenses.”
Grammar and usage: no point in one without the otherOne caveat: when teaching it was my ambition to teach grammar and usage together. In other words, if I found myself teaching a sentence that was grammatically correct but I could never imagine anyone ever saying it in a meaningful context I would take a step back and reconsider what I was teaching.
PresentPresent tense. “He eats spaghetti.” Not very meaningful. In context: “He loves spaghetti. He eats spaghetti every chance he gets.” We call it the “present tense” but obviously it refers to the past and the future. The one time period “eats” does not refer to is the present. The aspect can be described as habitual, repeated, factual or stative.
Present continuousPresent continuous. “He is eating spaghetti.” No obvious meaningful context. Maybe Mom calls home to the nanny to ask what little Johnny is having for lunch. “Is eating” does refer to the present, but it also refers to the past and the future. In fact, in the real world we most typically use the “present continuous” to indicate the future: “I’m seeing the doctor tomorrow.” The important issue is it’s aspect: it signals something continuing or in progress. The concept that students will eventually have to grasp is the difference between a “repeated” or “habitual” action and a “continuous” action. It is difficult to come up with an absolute, teachable distinction between these aspects, but the most obvious distinction is that a continuous action can be interrupted. (Think about it. We all think we know the difference between a liquid and a solid but physicists have yet to come up with an absolute distinction. Exactly at what point is a solid ice cube considered liquid water? Same problem with continuous versus repeated.)
Present PerfectPresent perfect. “He has eaten spaghetti.” Can you imagine yourself saying this in the real world? Here you would really have to stretch your imagination to come up with a meaningful context. How about: “He has eaten the spaghetti, but there is some lasagna left.” This “verb tense” drives Francophones crazy because there is no equivalent tense in French, but the structure (verb “to have” + past participle) is exactly the same as the simple past in French—but it’s not the simple past in English. Again, we call it the “present perfect” but it refers to an action that has taken place in the past. The concept that needs to be gotten across to students is the answer to "what does 'perfect' mean?" The perfect aspect implies a time frame within which the action happens (not the action itself) that is “perfect” or “complete” or like a circle or at least has a beginning point and an end point. The implied time frame extends from some time in the past to the moment of speaking, and the action occurs at some unspecified time within that “perfect,” completed time frame.
I have presented the following scenario to try to get across the meaning of the “present perfect”: John wants to ask Mary out, but she wants to politely, indirectly decline. John asks “Would you like to see Star Wars with me tonight?” Should Mary say: a) “I saw it.” or b) “I’ve seen it.”? Native speakers will recognize “b” (the present perfect) as the correct answer but will likely be at a loss to explain that the present perfect is used to signal an action in the past (Mary’s seeing the movie) which touches the present (Mary’s declining John’s invitation). Adverb phrases like “so far,” “already,” and “up until now” are the strongest signals that the present perfect is required.