They are very close in meaning, particularly in your example, because there is no real request or condition attached. The difference is important in conditions. If you happen to... has an element of luck, or perhaps, a request that can be fulfilled if it is convenient. It makes the condition passive. If you happen to pass a shop on the way home, you could buy some milk literally means 'if it occurs that you pass a shop...' or 'if your path home takes you past a shop, you could...'. Implied is 'i do not expect you to change your route just to fill this request' Whereas, If you were to pass a shop on the way home, you could buy some milk I think that if you choose to pass a shop, that would make it possible for you to buy some milk....'. Implied is 'please consider changing your route so that you can do this' Both mean 'please buy milk on the way home. The first is a softer request, not wanting to cause difficulty.