1.When the postman (to arrive), my father (to have) a shower. 2.While my brother (to study), my grandma (to make) some muffins. 3.They (to play) tennis when it began to rain. 4.We (to go) to the movies last night. 5.The thieves (to come) into the house while the man (to sleep).

1. When the postman arrived, my father was having a shower. 2. While my brother was studying, my grandma was making some muffins. 3. They were playing tennis when it began to rain. 4. We went to the movies last night. 5. The thieves came into the house while the man was sleeping.

Если сравнивать Past Simple и Past Continuous, то первое обозначает одномоментное, внезапное (suddenly), непродолжительное, краткосрочное действие, второе - наоборот, продолжительное.