Could you tell me if "la" and "a" in above sentences can be replaced by other modal particles?
Yes, that would be the case more often than not, but as you can see, there are no strict rules about "了" as it's used in so many ways.Thanks for the advice!
But as I understand it, the one who is being uttered this sentence in the second sentence of mine has not yet done shopping but probably is being headed for shopping now. Can we still use 了 in that sentence? I worry that using 了 in that sentence might mean "You have already gone shopping?"