I (talk) on the phone when I (understand) that I (forget) to lock the door.
I was talking on the phone when I understood that I forgot to lock the door.
While she (paint) a picture, her cat (jump) on the table.
While she was painting a picture, her cat jumped on the table
He (fix) his bike when the tire suddenly (go) flat.
He was fixing his bike when the tire suddenly went flat.
As I (write) an email, the computer suddenly (freeze).
As I was typing an email, the computer suddenly froze.
