the chair is comfortable to sit on为什么要加on?

the chair is comfortable to sit on为什么要加on? the chair is very comfortable to sit in 这个地方为什么要用in而不用on There is a comfortable chair to sit on .为什么加on The chair is too dirty to sit on.为什么要加on呢? The chair is comfortable to sit on/还是to be sat on? chair is comfortable to sit on 还是in ? the chair is soft to ____ 为什么要用SIT ON 不是BE SAT ON sit on the chair the chair is too hard to sit on.改为同义句 the chair i Sit on your chair!Now the chair is ______ you. Pam is on the ship.She sit on the chair. A dog is on the chair A.sit B.sitting C.siting D.to sit