For converting between two systems you need to know relations. For example 1 yard is 0.9144 meters. So every other number of yd you need to multiply by above number to get meter.
All metric units of a particular type, ie lengths, areas, volumes, weights, etc are convertible with scales of 10, 100, 1000, and so on. The size of the unit in question is determined by a prefix,...
Well, the only time you'd use this is in displacement of an irregular solid to find its density. In that case, if a liquid got displaced 200ml, you can convert it to cm3 like this: 200ml=200cm3