What is the grammatical difference between べきは and べきことは


I usually see べき nominalized as べきは. Is it grammatically correct or just a
casual way of saying? How is it different form べきことは?




べきは is a more pompous, not casual, way of saying べきことは.

  * 今やるべきことは待つことだ。  
今やるべきは待つことだ。  
What we should now is wait.

べき is the attributive (連体) form of べし, and using it like a noun is basically
an old way of speaking Japanese, although this is found in many patterns of
modern Japanese:

  * [Plain Verb followed by がよい](https://japanese.stackexchange.com/q/64661/5010)
  * [How can （の）目指すは be grammatical?](https://japanese.stackexchange.com/q/12538/5010)

That said, べき is already a stilted word, so the difference between べきは and
べきことは is not very large.




べきは is a more pompous, not casual, way of saying べきことは.

  * 今やるべきことは待つことだ。  
今やるべきは待つことだ。  
What we should now is wait.

べき is the attributive (連体) form of べし, and using it like a noun is basically
an old way of speaking Japanese, although this is found in many patterns of
modern Japanese:

  * [Plain Verb followed by がよい](https://japanese.stackexchange.com/q/64661/5010)
  * [How can （の）目指すは be grammatical?](https://japanese.stackexchange.com/q/12538/5010)

That said, べき is already a stilted word, so the difference between べきは and
べきことは is not very large.

