How do you say this in Korean? how to say i love you in korean?

사랑한다 is formal non-polite(impolite, informal) used to the same age(friend), someone younger than you, the children
(high formality / low politeness)

사랑해 is informal or casual used to the same age(friend), someone younger than you, the children, lover, wife & husband
(low formality / low politeness)

사랑해요 is informal (but still) polite used to olders, seniors, boss, lover, wife & husband, parents
(low formality / high politeness)

사랑합니다 is polite(more formal) used to olders, elders, seniors, boss, army, wife & husband, everyone
(high formality / high politeness)

formality: formal / casual
politeness: polite / rude