Past Simple or Past Continuous Tense

I ...(to write) an essay when my friend telephoned me.

Sam was walking his dog when he ... (to see) his classmates at the bus stop.

Nelly ...( to find) a very interesting article while she was preparing her project.

We ... (to talk) to the librarian when our teacher came up to us.

The children ... (to play) in the yard when it suddenly started raining.