"Korean is not difficult."
Translation:한국어는 어렵지 않아요.
The dictionary form is 어렵다 - if you use it to modify a noun, it becomes 어려운. If you use it for a negative statement, it becomes 어렵지 않아요 as you see in the sentence. You simply omit the 다 from the stem, and add -지 않아요 to make it negative in a casual polite form. Beware that 어렵다 is an irregular verb, that's why the form 어려운 lacks that ㅂ.
They're both forms of the verb 어렵다; there are no adjectives in Korean, but any verb can be modified into an adjectival form and precede a noun in a sentence.
You could say "Korean is not a difficult language", 한국어는 어려운 언어아니요; the focal verb of the sentence then becomes "is a language", rather than "is difficult". I hope that helps a little, I'm not great at explaining it.