I'm a US native speaker. When I was twelve, my English teacher in school called the word "separate" a "spelling demon," one of many. Even native speakers find it difficult to remember how this word is spelled. The problem is that the second and third sounds are indefinite "schwa" sound that could be written as almost any vowel. There are so many ways to mis-spell it! If you didn't know, you would probably guess "seprit" because that is close to the way it is pronounced. Other famous "spelling demons" include "parallel," "harass," "embarrass," and the three words "there," "their," and "they're."