would is used to say about determination like " i would finish this work "  and another usage is the that is to express past habitual actions "i would go on foot to school when i was studying 8th standard" like that. and could is used to say about past ability "i could give the seminar infront of 100 members" and could is used to make polite expressions "could u please move a little bit " and when we want to express the indirect speech also at the time COULD is used. anthony said, "i can do this work". anthony said that He could do that work. like that  and so many expressions and usages are there with could.