1. when I was passing the street, I saw Emily. 2. when I saw Emily, I was passing the street. In one, you are explaining what happened WHEN you were passing the street. (ie. you saw Emily) In two, you are describing WHAT you were doing (ie passing the street) when you saw Emily. 1. When I had seen Emily, I passed the street. 2. When I passed the street, I had seen Emily. In one, you are describing WHAT you had done when you had seen Emily. (you passed the street). In two, you are describing who you saw WHEN you passed the street (you saw Emily)

Check your grammar book for these topics: - when (referring to a moment) - When she called, I was cooking supper. - while (referring to a period) - While I was cooking supper, she called. - past perfect - She told me that she had had a hard day. Grammar in Use Intermediate by Murphy (Cambridge) is an excellent grammar book with explanations, exercises, and answers.

"passing the street" is very confusing. Perhaps you mean crossing the street? Walking along the street? If you change your examples, people could help you with answers.

'passing the street'? what do you mean? walking along the street?