It's OK to use words like "look" and "see" when you're talking with someone who is visually impaired or blind. It's the way we speak naturally, and it's not generally something that people will take offense to.
No, I do not know sign language. People who are visually impaired might have a hard time with that. I do know braille, though.
Visual impairment or blindness isn't a black and white sort of thing. Most people who fit into the category have some usable vision. The kind of vision they have depends totally on the particular case. People using white canes may very well have some vision - the cane is to let others know that someone has a visual impairment, as well as to provide feedback about the environment.
People can not harm their vision by using it. Of course, using it to stare at the sun will be detrimental, but general use is not going to do damage.
People without a visual impairment can not know what it's like to be blind by wearing a blindfold for ten minutes, an hour, or even a day.