when we answer a question if some one comment on my answer ( on the next login SU show us the comment(by the blue envelope)) but what if we comment on a question or comment other one answer how can we alert by SU that some one answer or say something about our comment(in this way the envelope don't become blue).
You will receive a notification only if someone answers your comment by using a
@yourname comment reply.
Not everyone uses it though, so I suggest you use it even when you don't need it. For example, even if you know the person who posted the answer you comment on will get notified, use the
@hisname when you write your comment, to suggest it's the way he got notified, so he will use it too (not needed with a regular user though, but it can help with new ones).
You should see the envelop next to your name become blue(SU), Red(SF)... and so on.