( a ) Mary walked downtown yesterday.
( b ) I slept for eight hours yesterday.
The SIMPLE PAST is used to talk about an activity or sitution that began and ended at a particular time in the past ( e.g.,yesterday, last night, two days ago, in 1999), as in ( a ) and ( b ).
( c ) I sat down at the dinner at 6:00 p.m. yesterday. Tom came to my house at 6:10 p.m. I was eating dinner when Tom came.
( d ) I went to bed at 10:00. The phone rang at 11:00. I was sleeping when the phone rang.
( e ) When the phone rang, I was sleeping.
( f ) The phone rang while I was sleeping.
The PAST CONTINUOUS expresses an activity that was in progress ( was occuring, was happening ) at a point in time in the past ( e.g., at 6:10 ) or at atime of another action ( e.g., when Tom came ). In ( c ) : eating was in progress at 6:10 ; eating was in progress when Tom came.
When = at that time.
While = during that time.
( e ) and ( f ) have the same meaning.
FORM : was/were + -ing.