1-you are trying on a jacket in a shop. you look in the mirror and you dont like what you see.
what do you say to yore frind?
It doesn't look very good, does it?
2- you and a friendarw wallking over a small wooden bridge. the bridgeis very old and some parts are broken.
what do you say ?
The bridge isn't very safe , is it?
plase tell me why we use It doesn't in (1) and isn't in (2) what's the diffrent??
Do is a strange but useful verb. It's about activity and often gets combined with other verbs to make them more active: How does it look? She did not sound good. How did it end? Did it end well? Yes, it did end well. How did I do?
Is is simply about a state of being. It sounds passive.
You could rephrase either sentence. To make the first more passive, you could say: "It isn't very good looking, is it?" To give the second a more active sound, you could say: "The bridge doesn't look safe."
So the difference is really just about tone. You probably can't hear the difference yet, but if you continue your English studies for a just a few more years, you will.
[I edit copy and have tutored college writing.]