sit , sit down

[melo]

I would like to know how and when to use SIT or SIT DOWN

 

[Anglika]

Sit = be in a position in which the lower part of the body is resting on a seat or other type of support, with the upper part of the body vertical:

>> We sit on the chairs in the dining room.


Sit down = to move your body to a sitting position

>> When I come home, I sit down on the sofa to watch TV

They can really be used interchangeably.