Read the following sets of sentences and say how the meaning changes. 

1 a Paul had left when Sally arrived.
   b Paul left when Sally arrived.

2 a Annie was locking the door when she heard a strange noise outside.
   b Annie locked the door when she heard a strange noise outside.

3 a James read the book last night.
   b James was reading the book at 10 o'clock last night.

4 a We were eating when they arrived.
   b We had eaten when they arrived.