i know it's out of nowhere but do you have a difficulty pronouncing Korean words? I have many foreign friends studying Korean, but many of them don't seem to know this website. http://www.forvo.com/languages/ko/ if you add words you don't know how to pronounce, Korean native will pronounce them for you.

Oh my god!

There are So many people who knew about korean. I really surprised about this. I am korean student and I think 본인, 당신 are not have much difference. It just relate about a situation when you use.

I really hope you guys understand and change my grammatical mistake. Thanks for reading it.

I think it's very unusual to call a woman 선생님 unless she's really a teacher/doctor.

선생님 can be used to every man ( above middle age, not to young man) although he's not a real teacher.

당신 is not used well in conversation as you said. only to wife/husband or when you're mad like i mentioned above.

사모님 itself means literally a wife of a teacher, so it's not ok to calll an unmarried woman. But if she is in middle age, it can be used.

So to younger age like 20~30 almost same age with you, 본인 can be used if you don't have any relations like  학생/학생분/환자분/고객님/선배님.

I use 그쪽 also instead of 본인 when i don't even know his/her name. (If I knew I said ~~씨/~~님)

본인께서 알아서 하시죠 > 그쪽이 알아서 하시죠.

Since 그쪽께서 sounds a bit weird. this case 본인 + 께서 +~~시다(존경표현) is best if you don't know anything about the person.

Thanks you for the insight!  Can I call a woman 선생님 as well?   선생님 means teacher but I guess it's ok to call every 선생님.  

I tried using Google translate.  Google uses "당신" which sounds weird because I thought you only called your husband or wife that. 

Is it ok to call an unmarried woman 사모님?  

당신 can be used in polite way but i think it's mostly in written method..

like in letter we can say, 당신께 감사한 마음을 담아 보냅니다.

in church the lyrics of ccm is 당신은 사랑받기 위해 태어난 사람. //

당신 also can mean honey,(wife/husband) 당신 오늘 늦게 들어와? /honey, are you late tonight?