〜にくい
Hard to do somethingmachine translation
FormationVerb ます-form (drop ます) + にくい
〜にくい is the mirror image of 〜やすい: take the verb ます-form, drop ます, and add にくい to say the action is "hard to do": 読みにくい (hard to read), 覚えにくい (hard to remember), 飲みにくい (hard to swallow).
Like 〜やすい, the verb + にくい combination conjugates as an い-adjective: 使いにくかった (was hard to use), 読みにくくて (hard to read, so…).
Note: 〜にくい describes difficulty due to the nature of the thing, not "impossible." If you want to say outright that something "can't be done," use the negative potential form (読めません), not 読みにくい.
Examples
この漢字は覚えにくいです。
This kanji is hard to remember.
この薬は大きくて飲みにくいです。
This pill is big, so it's hard to swallow.
彼の字は小さくて読みにくいです。
His handwriting is small and hard to read.