Like 易（やす）い, にくい is an い-Adjective that is regularly attached to the ます stem of verbs. However, instead of meaning ‘easy to (A)’, it means ‘difficult to (A)’. (A) will always represent the verb that comes before にくい.

• この駐車場（ちゅうしゃじょう）は狭（せまい）いから止（と）めにくい。
  This parking lot is very cramped, so it is difficult to park.
• 今日（きょう）は具合（ぐあい）が悪（わる）いから呼吸（こきゅう）がしにくい。
  Because I am not feeling well today, it is hard to breathe.

The nuance of にくい is that (A) is a task that is difficult to do, because of the required skill level, or similar factors. This is different from づらい, which focuses more on a task that is difficult due to being unbearable/hard to endure for some reason.

• 私（わたし）には英語（えいご）の「Literally」という単語（たんご）がとても言（い）いにくい。
  For me, the English word ‘literally’ is very hard to say. (Hard due to the individual's skill level)
• お前（まえ）には本当（ほんとう）に言（い）いづらいけど、お前（まえ）のギターを壊（こわ）した。ごめん。
  This is very difficult for me to say to you, but I broke your guitar. I'm sorry. (Hard because the speaker knows that telling the listener will cause a negative response)