Hard to... ～にくい

If "easy to" is easy to use (see above) then you would think that "hard to" would be hard to use.  Well, they had to go and make it easy as well.  Usage is the same as -yasui (easy to)

たべ ます tabe masu [to eat]　　たべ にくい tabe nikui [hard to eat]

 If you know the -masu form of the verb, you can easily drop the -masu and add a nikui.  

わかり ます wakari masu [to understand]　　わかり にくい wakari nikui [hard to understand]