Since the discussion of a very practical matter of teaching tenses has already gone philosophic, let me add another two-cents worth.  Few have mentioned the distinction between time and tense.  The latter is the grammaticalization of the former.  

Let me explain grammaticalization as follows.  In some European cultures the language learner must come to know the gender along with the name of any inanimate object.  Most probably anciently these objects were conceived of as having a natural sex.  In other words the Indo-European group of languages (including Old English) grammaticalized sex in the world as gender in their language.   Nowadays the gender of nouns in the derivative languages has little more than grammatical consequences.  In English little is left of this grammatical gender.  Note that we still view ships and lighter watercraft as female, and refer to them with the feminine pronouns.  

Now when we want to use a verb in English, we must use some grammaticalization of time.  There are grammatically only two tenses (present/past).  These are the grammatical forms.  As far as time is concerned there are many ways to express it.  Every sentence has tense, but not every sentence is meant to be have place in a particular time.  The various forms of the verb phrase contained in the auxiliary express different kinds of attitudes the speaker takes to the action, state, or situation expressed by the verb.  These grammaticalize as mode (optative, obligative [moral or social], potential, etc.), aspect (perfective, progressive, etc.), and voice (active, passive).  Some of these grammatical devices may or may not effect the sense of the verb with respect to time.  ["Present-perfect" is not a tense in English.  It is the combination of two grammatical devices: present tense and perfect aspect.  Its sense is various. ]

It seems to me that when we teach language, we should help the student make the distinction between the sense the writer wants to convey and the devices that the language has to express the sense.  By necessity the devices are deeply rooted in tradition and convention.  Our sense of the world is a growing expanding tree of knowledge and experience.  We should teach both aspects of language, but please let's not ignore or blurr the distinction.  

Bruce Despain
Volunteer ESL tudor

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