The only difference which I found by now is, that if you would like to say "some of.." you have to use "manche", e.g.: You just cannot work together with some of the colleagues -> Mit manchen Kollegen kann man einfach nicht zusammen arbeiten. But I'm not native, so maybe there is some other difference.

Many apples would be "viele Äpfel". "Manche" is less than "viel".

past tense would be 'Sie aßen manche Äpfel'