I love you,
in brief, 사랑해 [Sa-lang-hae]
full sentence is 나는 너를 사랑해 [Na-nun / Nul or No-reul / Sa-lang-hae]
나는 means 'I'
I miss you
in brief, 보고싶어 [Bo-Go-Si-Po]
if you say , 보고싶어 죽겠어,[Bo-Go-Si-Po Juk-Gue-Sso]
it has stronger feeling than short one.
– 당신을 사랑해요 is technically correct and is quite polite.
당신을 사랑해 is being used usually by middle aged and older people.
and, young lovers rarely use 당신을 사랑해. it may sound a little funny, if they use it
– I want to be with you
나는 당신과 같이 있고 싶어요 ( 싶습니다)
[Na-Neun / Dang-Sin-Gwa / Ga-Chi / Ee-Ggo / Si-Po-Yo (Sib-Sseum-Ni-Da)]
this is too polite, I think.
If your fiancee is youger than you, you'd better say this
너랑 같이 있고 싶어.
[No-Rang / Ga-Chi / Ee-Ggo Si-Po]
– I will be there soon
[Got / Gal-Kkai]
it's informal way.
– why don't you call your fiancee '자기야' [Ja-Gee-Ya]
it means like "honey, darling", and is used between young couple.