There is the accent symbol
> to mark loud note (relatively neighboring), but how should I mark soft note?
(It's sems logically to mark it with
<, but I never seen this in real sheets.)
What you are looking for is anti-accents, also called ghost notes:
(Image taken from the wikipedia.org page on "accents".)
The left one is slightly softer than other notes, the one on the right is much softer, and the one in the middle is, well, right in the middle.
Ghost notes are often notated with an
x instead of a circle for the notehead.