The phrase "a challenge to understand" is correct and usable in written English.
It can be used when describing something that is difficult to comprehend or grasp.
Example: "The complex theories presented in the lecture posed a challenge to understand for many students."
Alternatives: "difficult to grasp" or "hard to comprehend".
It is a challenge to understand what might have happened without more explanation from Mr Piketty.
But that is what makes the history of Prohibition such a challenge to understand.
"It's really a challenge to understand why these things are not wreaking havoc on the plant," Dr. Herre said.
It has been a challenge to understand the negative fouling resistance during the induction stage of many crystallization fouling processes.
It is a challenge to understand the shows' narration over the site's aging loudspeakers, but the general theme is clear: Essentially, good guys shoot bad guys.
It is a challenge to understand nonlinearities around the plasma frequency, where the interplay between the unusual spectrum and the nonlinearity of the JPWs is most pronounced.
Although all three have already been successfully addressed using experimental evolution in microbes, cancers provide a new challenge to understand how predictions derived from simpler biological systems translate to a more complex one.
It has been a long challenge to understand the equilibrium and the dynamic phenomena (e.g. chemical reactions) at the electrode/electrolyte interface in a unified theoretical framework.
Such cryptic signatures of selection have been also found in the bovine genome, posing a considerable challenge to understand the biological consequences of artificial selection.
It is still a great challenge to understand the nucleation and growth mechanism of bimetallic nanocrystals.
And so a greater challenge to understand what potential hazards exist within them".
