Hello? is there anyone know how to build a model like in the picture below?
Draw a bottom curve, draw a top curve, loft, cap, done.
how about if i make a box first? how can i change the flat surface into curve surface?
This is the last approach I’d take.
Build a box, explode it, rebuild surface with x>5 control points, join surfaces again, turn control points on, start dragging them to get the shape you want.
This is far less precise method.
ok.thank you. will try for it.
yes build a box, and maybe try ‘CageEdit’ command, add more points, push and pull points as needed, then explode, delete top and bottom, its a bit dirty but may get you to what you need 
edit- if you want to keep it a box no need to explode and delete top & bottom