文法與語氣的關係 - 翻譯

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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