For example, in English we would use the passive for a sentence such as "This castle was built 700 years ago," and the same would be true in Japanese ("このお城が七百年前建てられました。"): so far, so similar. However, the Japanese also tend to use the passive in other places where English speakers would not. Suppose someone stands on your foot in the bus. In English, you might say "Someone stepped on my foot." Grammatically, you can make that same sentence in Japanese ("誰かが私の足を踏ました"), but it sounds pretty unnatural. More likely you'd say "My foot was stood on" ("足が踏まれた"), leaving the "somebody" out altogether.
Of course, that would sound slightly unnatural in English - but then, why don't we also say, "Someone built this castle 700 years ago"? What are the rules in English for determining when the passive is used and when we use a "dummy" marker like "someone"?
I'm sure linguists have thought about it, but I don't know what their answers are.