used to express anxiety and uncertainty about an untoward event

• 这事别是他干的。

  Could this be his doing?

• 下雨了, 他别是不来了。

  It's raining; I hope this doesn't mean he won't come.

• 别是忘了告诉他了, 那可怎么办？

  Could we have forgotten to tell him? What will we do?