我不是故意做的，那是个意外。

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

他不是故意拿那本书的；那是个真正的错误。

He didn't take the book ___on ___purpose; it was a genuine mistake.

我不是故意撞到墙的；天太黑了，我只是不小心走了过去。

I didn't walk into the wall ___on ___purpose; it was dark and I just walked into it.