Q. When do you use "Sumimasen" as "I am sorry"? And when do you use "Gomennasai"?
A. Like Question Vol.5 (difference between "Sumimasen" and "Arigatou"), these two phrases are a matter of personal preference whether to use either "Sumimasen （すみません）" or "Gomennasai （ごめんなさい）." There are still a few situations I can tell you about.
- "Sumimasen" is a little more formal than "Gomennasai."
- When you apologize to the higher or the senior, "Sumimasen" is used in general.
- Among the family members or close friends, it is common to use "Gomennasai." "Gomen ne" or "Gomen" can be used in a more casual case.
- Older people tend to use "Sumimasen" more than younger people.
Both "Gomennasai" and "Sumimasen" are used when you have made some mistakes or inconvenienced someone. "Sumimasen" is also used when expressing a feeling of gratitude, but "Gomennasai" cannot be used in such situations. Also, "Gomennasai" can be used when apologizing to someone with whom you have a close relationship. But when speaking with superiors or people with whom one is not very close, "Sumimasen" or "Moushiwake arimasen" are used instead, since "Gomennasi" can have a childish ring to it.