Hello Shun
First of all, I should qualify point 5 in my last post: you might not expect to say "I go to work by train" while on the train, but you might say "I go to work by train every day" during a discussion with a fellow traveller. So I had better withdraw that part of it.
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As I have pointed out, "custom" is only a meaning, and it has past custom, present custom, and future custom. Therefore, custom cannot explain tense.
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Again, I'm sorry if I've misinterpreted your comment; but I would say that although we label e.g. "I go" in "I go to work by train" as "present tense" (i.e. it does not have a past tense inflection), its aspect is "habitual", i.e. it expresses a composite past-present-future context. If the speaker wanted to limit the context in some way, he would have to choose another form, e.g.
1. I used to go to work by train.
2. From now on, I'll be going to work by train.
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Furthermore, if according to customs, my I ask what is the difference between the three customs below?
Ex1: I walk 3 miles every day.
Ex2: I am walking 3 miles every day.
Ex3: I have been walking 3 miles every day.
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I would call the difference here one of focus:
Ex1: This is a neutral statement of a regular or habitual occurrence.
Ex2: This draws attention to the nature of the routine.
The present progressive conveys an action in progress; it does not in itself suggest habit (unless habit is part of the verb's meaning). Thus (to borrow Riverkid's distinction) when the present progressive is used for a regular or habitual action, it expresses "provisional routine" or "the possibility of change".
So your Ex2 might occur in this context, for instance:
3. I'm walking three miles every day at the moment; but when my knee gets better, I hope to do ten.
Ex3: The present perfect progressive conveys a action in progress, which began in the past, and has some relation to the present. Again, it does not in itself suggest habit.
When used in the context of a regular action, it therefore again imports a sense of "provisional routine" or "possible change". You might use it in this context, for instance:
4. I've been walking three miles every day, but my doctor says it's making my knee worse. So I may have to stop.
(I should add that the present tense and perfect/progressive aspect operate in many different ways. The uses I mention here do not exclude other uses.)
MrP