～にくい

にくい means "hard to do": the action takes effort, or the thing resists being done. It goes on the ます-stem of a verb, and the result is an い-adjective.

The verb is in the ます-stem, never the dictionary form.

• × 覚えるにくい → 覚えにくい

The thing that is hard to deal with becomes the subject of the new adjective, so it takes は or が where the plain verb would have had を.

Conjugating like an い-adjective

にくい changes the way たかい高い does: にくくない, にくかった, にくくて, and にくい before a noun.

Things that don't happen easily

With a verb that nobody does on purpose – break, burn, get dirty – にくい describes a property: the thing does not easily do it. This is the use you see on product labels.

にくい or づらい

づらい also means "hard to do", but from the point of view of the person who has to do it: it is uncomfortable or painful for them. にくい is the neutral choice, and the only one for the properties above.

The opposite of にくい is やすい, "easy to do", which attaches the same way.