케이티(와 함께 한 날)~

어제 나는 케이티(네) 집에(서) 잤는데, (아주) 재미있(었다)! 우리(는) 많은 영화를 봤(다)! 먼저, 우리(는) "죽음의 고등학교"라는 애니메이션을  보았고, (그 후) 차와 나쵸를 (먹었다). (좀 지루해졌을 때)  (난) "(한국) 영화 보자!"다고 말했(다)! 그래서(우리는), 고사(와) 첼로 (그리고) 여고괴담5을 봤(다). 너무 무서웠(다)! (전부 벌써 본 영화들 이였지만 그래도 난 또 봤다). 아직(도) 무서웠(다). 나는 (새벽) 3시 까지 자지 않았(다)! 오늘 너무 피곤(했지만) (상)관없었(다)! 케이티(와 함께 즐거웠다).

what the above teacher has written would work when you are speaking to your friends using a script. 
But by the way you wrote the paragraph out in english, it sounds much more like a written diary so, what I have here is a typical korean school diary version of what you said, quite heavily edited but still trying to use 90% + of exact word choices you made.

I assume you learned most of your korean by watching movies and dramas, which I agree is a great way to learn a language; however, as you probably have noticed, there are downsides to learning a language that way and it is that you have either hard time learning or getting used to making use of the formal form of the language.

In korean written language, you never use 반말 (colloqial, common, informal talking).You wouldn't write " I hated that book because it was sooooo not legit with that dude in the story being hella retarded. In conclusion, the author is a $h!!@ty writer and this book is a piece of crap," in your essay, would you? I suggest when you are writing out paragraphs, you should get used to placing 다 instead of 어 at the end of a sentence. Also, you should definitely study hard in learning the 존댓말 (formal).

지원 화이팅!

케이티랑 같이~

어제 나는 케이티의 집에(서) 잤는데 우리는 (매우) 재밌(었)어! 우리(는) 많은 영화를 봤어! 먼저, 우리가 애니메이션 (중) "죽음의 고등학교"라는 (영화를 보고) 그때(그리고 or 그후에) 차랑 나초를 받았어(먹었어). (내용이 지루하여) "한국어영화를 보자!"(라)고 말했어! 그래서, 고사랑 첼로랑 여고괴담5을 봤어. 너무 무서웠어! 그런데, 이미 다 영화를 봤는데 아직 지켜(???).. 나는 오전 3시 때까지 자지 않았어! 오늘 너무 피곤었어, 하지만 (상)관없었어! 케이티랑 같이 재밌어.

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Yesterday, I slept at Kate's house and we had alot of fun! We watched alot of movies! First, we watched an anime called "Highschool of the dead" then we got tea and nachos. When we got bored, I said "Let's watch Korean movies! So, we watched Death Bell (고사), Chello (첼로) and Whispering Corridors 5 (여고괴담을5) they were really scary! However, I already say all these movies but I still watched them. They were still scary.. I didn't sleep until 3:00AM! I was really tired today, but I didn't care! I had fun with Katie.