What’s the difference between 费解 vs 难懂?
费解 = have difficulty understanding (You can't think of any reason for that to happen)
令人费解 (make people have difficulty understanding) = incomprehensible
Example:

Your friendly dog suddenly attacks you is an incomprehensible event (you cannot think of any reason for that to happen)

Ignoring more guns lead to more gun violence  is an incomprehensible act (you cannot think of any reason for anyone to think like that)


难懂 = difficult to understand (something is too complex or confusing to be understood)
Example:

The Mandarin he speaks is difficult to understand because of his heavy Cantonese accent (you get confused)

Calculation of rocket trajectory to the moon is difficult to understand (too complex for a layman)



1.这句话让我很费解。 这句话让我很难懂。


这句话很让人费解。 这句话很难懂。


Better use 让人费解 (make everyone have  difficulty understanding) instead of 让我费解 (make only I have difficulty understanding) to express 'incomprehensible'


2.这真令人费解。 这真令人难懂。


这真令人费解。 这真难懂。


3.他的一番莫名其妙的话让我费解。 他的一番莫名其妙的话让我难懂。


他的一番莫名其妙的话让人费解。 他的一番莫名其妙的话十分难懂。


4.这本书比较难懂。 这本书比较费解。


这本书比较难懂(O)。 这本书比较费解 (X)。


费解 (have difficulty understanding) is rarely used alone, It is always 令人费解 (make people have difficulty understanding = incomprehensible)

令我难懂/令人难懂 (make me/ people difficult to understand) is not grammatical-- 令人 is a verb, 难懂 is an adjective