Circuit 1:

1-Rotational of potentiometer leads to change in resistance that leads to change in current pass through led so intensity of light increase by rotate potentiometer anticlockwise direction.
2-

Circuit 2:

1-We use two switches to can control led from two differant places.
2-YES.

Circuit 3:

1-DIM till capacitor charged.
2-Time of delay increase by connect two capacitors in series and more with parallel.

Circuit 4:

Answers

Circuit 5:

1-To decrease voltage on base of transistor.
2-We can use logic signals or ourselves.

Circuit 6: