THE STRING WITHIN A PEARL 
There was a sequence of pearls, connected by a string. Some people were curious about the use of the string, so they carefully cut off a pearl, together with the string inside, and study why we use the string.
What is the use of the string inside ONE pearl? Some persons concluded its use is A. Some others concluded B. However, they agreed that, despite some difference in opinion, they had approximately known the true use of the string.
And then a person who knew few precious things like pearls, came in and provided evidences to show the use of the string is neither A nor B. People thought he was making trouble and challenged him to tell the use of the string. The guy admitted he didn't know, but showed evidences that its use was not they had concluded.
People told him to be careful. If he couldn't provide an answer he had to accept either A or B, one of their conclusions.
What does the story say?
:P To examine a tense on one-sentence basis is like to examine a pearl cut off from a sequence of pearls, together with its string inside (like the tense inside one sentence).
What is the use of a string inside one pearl (= what is the use of a tense inside one sentence)?
The string can be well explained as we put the pearl back to a sequence of pearls (= the tense can be well explained as we put the sentence back to its paragraph).
TRUE USE OF TENSES To discuss the use of a tense on the basis of one single sentence, we will never see its true use. That is to say, we may murmur more years about the use of a tense on one-sentence basis, and the conclusion must still be a failure or ambiguity. 
:mad: To cover the failure or ambiguity, grammar writers secretly or hopelessly create more ambiguities to do it. This vicious circle results in numerous jargons and unfathomable theories, just for dealing with the problem a student inevitably encounters. Without knowing much, student must say thanks and leave, having got the 'answer' from us. This reinforces our confidence in creation of new jargons and grotesque methods. Now we have come to a stage that even experienced learners don't even know we are using jargons, borrowing definitions, and ignoring facts.
It is time for us to call for a halt. Using a paragraph to explain tenses is accurate, easy and painless. The choice of tenses is literally measurable in time. Most important, to tell the truth, this is the only way to find the true use of tenses.
:wink:
To explain a tense on one-sentence basis is like to explain the string inside one pearl. The more you look at it, the more confusion we will get. 
Linear Mode
