bemoan... look at moan..means cryin,..whn does one cry when dey loose something..when one looses something they express disapproval..
bemoan -- be mourn means express mourn -- express sadness
be+moan--moon->in olden days mothers used to show their kids the moon and used to tell some stories and feed them.now most of them are living in apartments, where there is no scope of showing moon and feed them, so they beat them
be+moan--moan sounds like maun(hindi word)(few minute silence in remembrance of someone.) in order to express our grief when someone dies we keep maun for few minutes.
BEAM+ON...the meaning is lament.....I didn't switch on the beam(light) of my car and because of that I had an accident hence I LAMENT...
bemoan has moan in it which means "A low, sustained, mournful cry, usually indicative of sorrow or pain"