When you touch the top tape and get called for it.
Yes. I'm not sure on all levels but I know that just this year the FIVB has changed the net rule to allow players to contact the net as long as it does not interfere with play. (Rule 11.3)
No, in club volleyball, you can touch any part of the net except for the top tape, and in high school and college, you cannot touch any part of the net.
no part of your body can touch the volleyball net
The volleyball may touch the net while crossing over it. A volleyball driven into the net may be played within the limits of the 3 team hits, so long as it indeed remains in play (typically: stays in-bounds and off the ground). If the ball rips the net or tears it down the rally must be replayed.
just don't touch the net