sofa is to lie down I think!...
Lie down
A sofa is also for sitting but it is typically larger and can seat multiple people compared to a chair.
a chair, a sofa, bottom, and bench.
On a hard chair
Sofa
You don't unless you have a chair or couch or sofa at home, and that's a different story.........
between the chair and the sofa
Your bottom. Sit-upon. Derriere. Tushie. The part you always sit on.What your bottom is sitting on depends on where you are. It could be a chair, a bench, a sofa, a stool, a rock, the ground, a fallen log, a straw mat, a throne (uncommon), and so on.
WHEN I SIT ON A CHAIR IT MAKES ME RELAX.
You sit ON a chair, you sit AT a desk.
a sit in is when you sit in a chair
cushion
It is on the chair not in the chair you do not say in am sitting in the chair although you do you saiy I'm sitting on the the chair, an dchairwould be like a stool if you havent got a back rest or anthing whereas if it is a fully arm chair then it is in the chair because you are sitting down with a cover surrounding you back!