Easy is the meaning of 簡単 in English. It is the adjective, so you must use easy as in something is easy. If you want to say it is not easy, you can put the un and make it uneasy which will mean not easy, difficult. Simple has the same meaning as easy. In Japanese if you say simple it means like it is plain but in English it also means that it is not hard and it is easy for you to do something. If you want to use the noun, not the adjective to describe, you can use the noun putting "the" before the noun.
日本 
