Not sure if you know this, but both of the examples you mention are the same in German: "..., weil es schwierig ist" and "..., denn es ist schwierig" both mean

Not sure if you know this, but both of the examples you mention are the same in German: "..., weil es schwierig ist" and "..., denn es ist schwierig" both mean "... because it's difficult", but "weil" and "denn" have different requirements for where the verb goes. And "ich lade euch ein" is also a mandatory splitting of "einladen", "to invite".