past progressive

[chum]

"I was driving when I saw Cathy" I know is correct how about

1. "When I was driving, I saw Cathy"

For me, when it comes to using "when" in past progressive,it should go with simple past like the first sentence...how about if I change it to " While I was driving, I saw her..Is this correct?Using while instead of when?and is it ok if i use that sentence in formal writing?Thanks!

[panjandrum]

Both are correct , and you may find some of us will say "Whilst" it means the same as while.

"When I was driving, I saw Cathy" (( Correct ))

"While I was driving, I saw Cathy" (( Correct ))