l have a 1997 mercury sable. when l close the door the interior lights stays on,why?

Check the dash mounted dash light dimmer. It also will set the interior lights to ON, even with the doors closed.

May be a timer that keeps them on for awhile, otherwise the dome light switch might be on, or a bad door switch, or a short to ground somewhere.

what they said check all the switches that are in sight

Check the push button on the door jam . l bet it looseor bent.

If the lights stay on with the keys in the ignition and in the run position or vehicle running..take some WD40 and lube the door latches.
The door ajar switch is sticking on one of the doors.
Lube all of them then open and close the doors a bunch of times untill the lights go out.