I'm going to address some of the most commmonly used (and most commonly misused) Korean terms in fanfics. Enjoy~

1) "name"-ssi

This term is used after a name such as Eunhyuk-ssi or Donghae-ssi. This term is equivalent to Mr., Mrs,, Miss, or Ms. in English, such as Mr. Smith or Mrs. Jones. One difference is that in English, or at least in the American English, you use Mr. or Mrs. and such on last names most of the time. In Korean, you tend to use it on the actual name. Of course, you can use it on actual names in English and on last names in Korean as well, but it's less common.

Example:  Lee Donghae      English: Mr. Lee      Korean: Donghae-ssi

Another difference is that there's different terms for male and female for English. And the term for women changes in English when they get married. However, there's isn't any of those in Korean.

Example:  Lee Hyukjae (Eunhyuk's real name)      English: Mr. Lee     Korean: Hyukjae-ssi

                Kim Taeyeon (Leader of Girls' Generation)    English: Ms. Kim or Miss Kim (Mrs. is for married women)

                                                                                 Korean: Taeyeon-ssi (even if she's married)

Finally, -ssi is used between people who are not close to each other. You don't use it to address your friends and certainly not members. One thing to note is that members might address each other with -ssi in broadcasted programs and such. The reason is that's just a rule to be followed in certain broadcasting companies.