"Yocreoqueestaclaseesmuydura."

I guess this means that 'hard' has the same physical and metaphorical meanings in Spanish as in English.

Personally i would say I think that this class is very hard

Wouldn't dificil be more appropriate in this sentence?

Why use que only to omit it in translation?

Yo creo already is I Think

adding Que implies You Think That... This class is very hard.