我无法打开这个文件，因为密码忘记了。

I ___cannot open this file because I forgot the password.

学生在教室里不能使用手机。

Students ___cannot use mobile phones in the classroom.

他不可能已经完成作业了，刚刚还在玩游戏呢。

He ___cannot have finished his homework already—he was just playing games.