2)
Waves can interfere constructively or destructively.
The points where a wave doesn't move are called nodes, and the wave interference at those points is destructive.
3)
A standing wave with minimum energy is created at a fundamental frequency f_1, and we can go up or down a harmonic by increasing or decreasing the frequency by discrete amounts of size f_1:
Each "loop" acounts for half a wave.
4)
Following the explanation given for the 3rd question, a 3-rd harmonic standing wave contains 1.5 complete waves, while a 6-th harmonic standing wave contains 3 complete waves.