Comming from a german language background, I am always uncertain about rules for the usage of commas in english. So could someone explain to me why there is no comma before "that" in a sentence like "we show that for certain expressions the usage of a comma is inevitable".

edit including suggestions in the comments below:

"we show that, for certain expressions, the usage of a comma is inevitable"

or

"we show that for certain expressions, the usage of a comma is inevitable"