Syntax-wise, Koreans and Japanese are very close. The sound systems are also close (e.g. no p/f v/b distinction). Kanji and Hanja are just Japanese/Korean versions of Chinese. Some words are shared and some nursery rhymes have the same melody. They even memorize the multiplication table with the same melody. In many ways, their languages and cultures have evolved with proximity for a long time, like thousands of years.

English and Korean are just two languages that would have almost no way of influencing each other directly for a long long time. They evolved independently, and once one is familiar with one language system, learning the other is just... painfully difficult in my experience.

If you see grammar errors in my comment, a Japanese person would very likely show similar mistakes in their English writings.