我不是故意做的，那是个意外。

I didn't do it ___on ___purpose, it was an accident.

她故意把门开着让猫进来。

She left the door open ___on ___purpose to let the cat in.

天太黑，所以我踩到了你的脚，但不是故意的。

It was dark, so I stepped on your foot, but not ___on ___purpose.