do violinist bow with the bow hair flat down or they only use 3/4 of their bow hair? which one is better

You mean do they tilt the bow when playing? I was taught to tilt the bow to the right when at the heel/frog of the bow, flat when I reach the middle and tilt to the left when I reach the tip... I think it also depends on the tone you want, but I'll let someone more experienced than me go into stuff like that.

I think you can have it so the bow is straight (and all the hair on the string) or have it tilted slightly (so the bow is toward your fingers) but still have the hair flat on the strings from the pressure if you want a bigger sound. If you want it softer volume, but still good quality, have it tilted toward your fingers, "use 2 hairs" as my teacher always tells us.

So it does depend on the sound you want.

ok, i think its better to play with the bow tilted because it sound much better. but i can't seem to keep bowing straight.

You're probably moving your shoulder back when playing - just make sure you move your lower arm from the elbow only in a forward diagonal motion and your bowing should start to get straighter.

It takes practice to get your bowing straight. make sure that the top of your bowing wrist is up and the knuckles of your hand lower then it when at the nut/frog end and that it is down and the knuckles higher then it at the tip end.
better yet, make your teacher show you exercises for this. they should, its EXTREMELY important