Their syntax and grammar is very similar. Korean and Japanese are both among the most difficult languages for an English speaker to learn, so I would imagine the reverse is true for Japanese and Korean speakers as well.

Consider the difference between English and Spanish vs. English and Japanese. If you look at a Spanish sentence, you can more or less translate it word for word to and from English so long as you know the vocabulary and verb conjugations. Japanese on the other hand has an entirely different grammar, uses the case (particle) system, has honorifics and levels of politeness, has entirely different parts of speech from english, is far less ego-oriented (I don't mean this in a judgemental way, just that objects are much more often the subjects of sentences)