how does a commutator in a d.c. generator work?

Question

i understand that it converts a.c voltage into d.c. but don't undrstand how

Answer 1

As the armature turns, the contacts rub against the brushes and to wires supplying the DC output. The contacts are staggerred such that the positive going voltage is always applied to the same brush. As the coil starts to go towards a negative voltage the armature rotates slightly more and the contacts switch polarity.
Have a look on How Stuff Works for a DC motor. Its commutator will operate in a similar manner to that in a generator.