PP vs PS
While he _____(sit) under a tree, an apple ____(fall) on him.
While he was sitting under a tree, an apple fell on him.
She was driving home when/while she saw the accident.
She was driving home when she saw the accident.
I meet an old friend when/while I was walking home.
I meet an old friend when I was walking home.
I meet an old friend while I was walking home.
When we ______ (sit) in the library, the alarm ____ (go) off.
When we were sitting in the library, the alarm went off.
She _____ (bring) some chocolates to the party.
She brought some chocolates to the party.