>これはつまり、"you still study it"の場合は、下記(1)ではなく(2)にするということですね。
>(1) I've studied English for four day and I'm still doing it.
>(2) I've [been] stud[ying] English [on my own] for [four] days [now] and I'm still doing it."
Yes, I would go with #2 in this case. Some verbs, especially that imply a currently active participation, sound better in the “have been ___ing” form. However, some verbs sound better in the “have ___ed” form. Note the difference in the following:
1. I’ve lived in Japan for five years now, and I think it’s time for me to move back to Canada.
2. I’ve only been living in Japan for five days but I love it here already.
#1 implies a longer period of time--as well as being at or toward the end--of involvement.
#2 is for a much shorter period of time that the speaker has been in Japan, and the implication is that he will continue to live there.
Going back to the two examples you give above, the other difference between the two forms is that if you said “I've studied English for four days,” without more, you would not know what happened after four days. On the other hand, by saying “I've [been] stud[ying] English [on my own] for [four] days [now],” one can infer that the speaker intends to continue studying.
>ということは、I've studied English for four days.は、次の(3)のようであればOKと言うことになりませんか？
>(3)I've studied English for four days but I stopped.
In this case, I would go with “I studied English for four days but stopped”. With simple sentences, I’d use the past tense: “I drove for four days to get from Los Angeles to Chicago.” These two sentences imply that the event occurred recently.
However, if someone asked you “Have you ever studied English?” you could answer by saying “Yes, I have studied English once, but it was only for four days and I was only in the fifth grade.” Here, the event took place many years previously.
It all depends on the situation. I can’t explain the rule because no one ever explained to me what the rules are, but I learned by reading and listening to others. Sorry it has to be more difficult than it needs to be.
