I heard if the bow is too tight it is bad and if it is too loose it is bad as well,why? And how to judge if it is not too loose or tight?

All advice/suggestions are welcomed.

I heard if the bow is too tight it is bad and if it is too loose it is bad as well,why?

As a rule too much of anything is bad. That's pretty much implicit in the meaning of too much.

The bow has to be tight enough to grab the string properly, but if you tighten it too much or fail to loosen it between playing or practicing sessions, the bow may warp.

ok, so as has been said you don't want the bow to curve in a "sad" face but in a smily face, this concave curve is like a spring, so that when you tighten the hairs it makes tension on them and keeps them taut, and when you loosen you obviously let the spring go. just like any normal spring a bow can be overstretched and then it won't work as effectively. this is why you loosen the bow after play to keep it springy.
a good way to check of your bow is tight enough is to wind it in a little and then place it on the string, if you press and the wood touches the string very easily then its too loose, if it is really hard to push down on the bow and it doesn't move then it is to tight. you want to be able to move the bow up and down with your first finger of your bow hand easily so that the wood can get closer and further away from the strings, but not so that it touches easily.
ok thats enough of a lecture! sorry it went on!
i hope that helps but you'll get used to where is easiest to play for you.

i think u got the point on how much to tighten it :P but remember to loosen it when you put it away too!!

oh........thanks everyone!

A simple way to tighten your bow enough so that it can produce a decent sound is simply by putting a pencil between the hair and wood part of the bow while its not tightened.

Continue to tighten the bow until the pencil falls, that's usually enough. Though I tend to tighten my bow a little more than that, its all based on your preference

lol its kinda a hard thing to explain its better to show... get your teacher to show you...

hi. i've been playing the violin for 3 years. You will learn! you need to tighten it enough that a pencil stuck inside will easily fall out if you tilt it. Although this technique worked for me, asking a teacher is the best way to go

Your little finger should fit exactly between the bow itself and the hair. Or, the space between the bow and the hair should be as wide as the bow. At least that's what I've been told...