The default amount of seconds for forward and rewind is 10 seconds with Left and Right keys. It sometimes is too many for me.

In SMPlayer, the settings can be changed at Preferences Interface Seeking.

In MPlayer, you can create a ~/.mplayer/input.conf with content as follows:

LEFT  seek  -1
RIGHT seek  +1
DOWN  seek -10
UP    seek +10
PGDWN seek -60
PGUP  seek +60

I don't want to override the default keys, but they don't work with modifier keys. When I press Shift+Left, MPlayer only knows it as only Left

The default settings in MPlayer already include Shift+Left and others, you can see the default keys and code. Unfortunately, they don't work for me.

The default configuration says:

13 ## You can use modifier-key combinations like Shift+Left or Ctrl+Alt+x with
14 ## modifiers Shift, Ctrl, Alt and Meta, but note that currently reading
15 ## key combinations is only supported through the video windows of X-based
16 ## output drivers (not in output windows of other drivers or in a terminal).

I have tried x11 or xv, but none of them could get the modifier keys working.