YOU SAID:

I am sitting on a chair

INTO JAPANESE

私は椅子に座ってください。

BACK INTO ENGLISH

I sit in the Chair.

INTO JAPANESE

椅子に座る。

BACK INTO ENGLISH

Sit in a Chair.

INTO JAPANESE

椅子に座っています。

BACK INTO ENGLISH

Sitting on a Chair.

INTO JAPANESE

椅子に座っています。

BACK INTO ENGLISH

Sitting on a Chair.