안녕 means hello!
You say 안녕 when you're talking to friends or someone of similar equal standing - you don't say it to your grandparents or someone you respect.
안녕 also means goodbye!
In Korean, a lot of meaning comes from our tone of voice and content - it should be obvious whether you're saying hello or goodbye.
Can you figure out how to pronounce 안녕?
잘 지내니 is how you ask, How are you doing? in Korean.
You only use this phrase when you're talking to a friend (like above) - in other words, only when you're speaking informally.
Can you figure out how to pronounce 잘 지내니?
사랑해 means I love you.
사랑해 is, yet again, informal.
Can you figure out how to pronounce 사랑해?