This is the 11063rd most frequent Spanish word.
duro
"Hard" or "tough."
Here, 'duro' means 'difficult' or 'challenging' in the context of describing the difficulty level of the exam.
Este examen es muy duro, pero estoy seguro de que puedo aprobarlo.
This exam is very hard, but I'm confident I can pass it.
In this sentence, 'duro' describes a physical property, meaning 'rigid' or 'solid', referring to the firmness of the ground.
El suelo está muy duro después de la helada de anoche.
The ground is very hard after last night's frost.
Here, 'duro' is used in an adverbial sense, meaning 'intensely' or 'with great effort', indicating the degree of effort applied in the situation.