难以置信 can be used as a predicate (这件事难以置信) or as an attributive modifier with 的 (难以置信的事). It often pairs with 令人 (make people feel) for emphasis: 令人难以置信.
memory
难 (hard) + 以 (to) + 置 (place) + 信 (trust) = hard to place trust in it. Literally 'difficult to put down belief'.
