<< Some modal verbs convey very different aspects of time depending on context:
When I was young, I could run five miles in thirty minutes. (past)
I could do that with my eyes closed. (conditional future)
I wouldn't do that right now. (conditional future)
I would never do that when I was in school. (past) >>
My reply: I don't think the notes in the bracelets fit the examples. "I could do that with my eyes closed" may be referring to a past case of playing piano, rather than a future.
Why are modal auxiliary verbs called "auxiliary"? It is because they mainly help. However, each of them has its own lexical meaning: WILL denoting willingness, CAN expressing ability, OUGHT TO saying an obligation, etc. The rule is, when they are not in the duty of expressing lexical meaning, they help other verbs to express a future action, that is, an uncertainty.
Now in your examples some of them are expressing the future, while some are still in their lexical meanings:
Ex1: The two boys return tomorrow to finish the work. (present certainty)
Ex2: The two boys will return tomorrow to finish the work. (present uncertainty)
Ex3: As you see, I can do that with my eye closed. (lexical meaning, present ability)
Ex4: I will not do it right now. (lexical meaning, present willingness)
If we keep an eye on these actions, next week we may see some contrasts. By next week, an uncertainty may become a certainty known to us, so the tense is different. We may in the following analyze the examples above, one by one, as looking back at them next week. Please understand now the time is past for all these actions above and we are doing the retrospect.
As for Ex1, since in retrospect the action is past, we use past tense to express it:
Ex1b: The two boys returned to finish the work.
==Because of this contrast, we have to put a name to its present form in Ex1. We therefore call Ex1 a Simple Present action. People fail to see time comes from contrast, so they sometimes claim the present is timeless. Now you even claim there is "no tense here". As for me, I have always pointed out the present exists because of a contrast with the past. As you agree there is past, you have to presuppose there is not-yet-past. What is the suitable name for not-yet-past, if not 'present'? Therefore, the tense in Ex1 indicating an action not-yet-past is the present tense.
As for Ex2, please understand that if we now take a guess at the future, it is an present uncertainty or present guess, rather than a future one. In future, we don't need to guess. That is to say, Ex2 is a present uncertainty:
Ex2: The two boys will return tomorrow to finish the work. (present uncertainty)
By next week, it has become a certainty, we use past tense to say whether it is real or not:
Ex2b: The two boys returned to finish the work.
Ex2b: The two boys didn't return to finish the work.
Ex2b: ?The two boys would return to finish the work. (incorrect if standing for a past certainty.)
== As we may see, we cannot keep the modal verb here, whether in past form or present form. This proves that, as the Future Tense, WILL cannot serve as a past tense. This is the most important feature to prove what is the Future Tense.
As for Ex3, since CAN in lexical meaning is not a Future Tense, its has its past tense COULD:
Ex3b: As you saw last week, I could do that with my eye closed.
== It is no more than a past and present contrast. Again, as you agree this is past, you have to agree there is not-yet-past, as in Ex3, which is called present.
As for Ex4, for WILL in lexical meaning is not a Future Tense, its has its past tense WOULD:
Ex4b: I wouldn't do it.
== A willingness in the past.
However, only WOULD and COULD can function as lexical meaning in the past form. That is to say, we don't have such a past form in other auxiliary verbs:
Ex4c: ?I might not do it. (incorrect if indicating a past)
Ex4c: ?I should not do it. (incorrect if indicating a past)
Ex4c: ?I must not do it. (incorrect if indicating a past)
== Actually, as the Future Tense, their past forms (might/should/etc.) also serve as Future Tense, yet with a weaker degree of uncertainty.
Because an auxiliary verb may be busy in its lexical function, so we have to keep a few auxiliary verbs in preparation, helping other verbs to denote a future. If WILL is busy, we use MAY, etc. However, as people cannot see the time flow as above, they claim that it is enough for us to use only one auxiliary verb in WILL to express the future. Later on, they even suspect if there is the Future Tense at all.
The Future Tense is much more difficult than Simple Past. As past time will not shift to present or future, the future will first shift to the present, and then the past. Without displaying the time flow as we do above, explaining the Future Tense is nearly impossible.
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