YOU SAID:
the green cat sat on the blue mat
INTO JAPANESE
緑の猫は青いマットの上に座っていました
BACK INTO ENGLISH
The green cat was sitting on a blue mat
INTO JAPANESE
緑の猫は青いマットの上に座っていた
BACK INTO ENGLISH
The green cat was sitting on a blue mat
You've done this before, haven't you.