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How to say 'I miss you' in korean language?

Bogoshipo (informal)
Bogoshipda (formal)

보고 싶어(요) (the 요/yo can be removed if you are close/friendly with the person)
bo-go si-po-(yo)

The structure VERB (base form) + 고 싶다 = I want to VERB, so adding 보다 (just the 보 part cos it's the base form of the verb) to 고 싶다 literally means "I want to see you", but it is most commonly used to mean "I miss you".

Below are some other ways to write/say it:
보고 싶다 bo-go sip-da
보 고 싶당 bo-go sip-dang (cute form)
보고 싶어용 bo-go si-po-yong (cute form)

Alternatively, you can add 너무/neo-mu (too much), 진짜/jin-jja (really) or 정말/jong-mal (really) BEFORE it to make it stronger, e.g. 너무 보고 싶어 - I miss you so much.