我不是故意做的，那是个意外。

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

我不是故意走进去的；天太黑了。

I didn't walk into it ___on ___purpose; it was dark.

她故意让门开着，以便让猫进来。

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