Thank you all for your replies to my thread. I really appreciate^_^ I've tried googling but I still feel confused. Maybe I put the wrong keywords. Like I just said, "sometimes" tense involves not only refering time, but also having connotations, say how politely you ask people or just ask without any emotion. This is why I brought this question up. Of course I know the importance of context, but in some cases (like explame 3) there are no context. I suppose there is some logic in a native speaker's mind when he/she choose to say that way or this way. For example, you can easily tell from this post that I'm not a native speaker. Maybe I'm wrong, just wondering/wonder/wondered..:P Your comments are very enlightening, but I still have a blurred pircture of it :~~ You know it because you just know it? --

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