Adding to @choco-addicted's great answer, there are a few more things that you need to note.
- '당신' is not a very broadly used word in Korean. The Korean language doesn't use the subject or possessive pronoun as often as English and usually it is implied. For example,
성함 (honorific for 이름) 이 어떻게 되세요? What is your name?
It sounds awkward if you use '당신의' before '성함' or '이름' as in "당신의 성함이 어떻게 되세요?" It is not grammatically wrong, but 당신 doesn't sound that honorific as it is also used to refer to the second person in argument.
- '당신' is also used as an honorific word for the third person, e.g., your grand father/mother, your father/mother. For example,
어머니는 당신이 아프시더라도 돈을 아끼기 위해 병원에 가지 않았다. My mother didn't go to hospital to save money even though she was sick.
You can use '그녀' in the above sentence, but it doesn't sound honorific.