In Korean, there is a defined difference in the language you use to speak formally and informally.
At its core, that distinction revolves around 요.
For some words, changing it from informal to formal is as simple as just adding 요:
For example, 사랑해 becomes 사랑해요.
There are a few rules to know, but formality is one of the few things in Korean that doesn't have a strict pattern, and so can only be learned by practice.
For example, 잘 지내니? becomes 잘 지내요?Never end in ㅏ요
Phrases like "예쁘다" end in ㅏ, and if we add 요, we end up with 다요 or some equivalent ㅏ요. This is incorrect - the correct way varies, but in this case, it would be "예뻐요".
Never use "너", "니", or "넌"
These mean "you" in various contexts - they're incredibly informal, and more for those you're familiar with, even among informal speech. Instead, we usually just use the subject's name: instead of "넌 예뻐," which means "you're beautiful", you would say "지민 예뻐 요", which translates to "Jimin, you're beautiful."No contractions
Korean has contractions - which we'll discuss soon.
When speaking formally, don't use them.
We'll expand on this when we discuss contractions, but simply put, avoid "넌" (from before), "해" (사랑햬 is an exception developed from common speech, kind of like slang), and others, of which we'll list as many as possible when we discuss contractions.
There are actually more than just two levels of Korean speech.
There's informal speech, which we call 반말 (literally, half-speak), polite speech, formal speech, and many more - some claim there's 7, others 9, others 3.
It's not that important to know the official names or exact tenses in modern Korean - it really suffices to just know informal speech and the formal speech we just learned.