In English, the word "on" is used more for when a person is standing on top of something. For example: "The girl is standing on the desk." There are, of course, other uses for on in English, but that one is the most common. "At" is used especially for location. "I was at the bank when the fire alarm went off." Another example would be: "I was at the park when it began to rain." "On" would not be appropriate for those cases. If you said, "I was on the bank when the fire alarm went off", that would make it sound like you were actually standing on top of the bank. So the correct word to use in the above sentence would be "at". :)