About Polite Language
Some interesting but really hard feature in Korean is polite language. In English, the language doesn't change a lot between situations like age difference, ranks in the group, friendliness, publicity of the situation, etc.
However, in Korean, the language, especially endings, changes dramatically because of those situations.
Let's say you want to say "I read a book yesterday".
It won't change if you speak to it your friend, kids, or the president in your country. And it won't change if you speak it in formal conference or casually in coffee shop. And it won't change if you speak it to someone familiar or unfamiliar. Sadly, it changes in Korean.
If you're talking to your friends, kids, it should be "나 어제 책 한 권 읽었어".
However if you're talking to the president or some older people, you should use "저 어제 책 한 권 읽었습니다." You can use "저 어제 책 한 권 읽었어요" to them if you're friendly with them.
Sometimes, the ranks between the people in the sentence matters, too.
If you're talking to your boss about the vice-president, then it's wrong to speak like "사장님, 부사장님은 잠깐 나가셨습니다." It should be "사장님, 부사장은 잠깐 나갔습니다". Vice-president is lower than the boss. So, you should remove politeness part in the language. It's one of the grammar that even native Koreans make error.
And the story doesn't end here. Sometimes, you should add some special endings or markers for politeness. Even you should use some polite version of the words like 편찮으시다 for 아프다, 계시다 for 있다, 진지 for 밥, etc.
Like always, I'll help you understand how to use them one by one.