You already know the rule: an essay is written in -ㄴ/는다, not -아요/어요. The TOPIK II course said so, and it is the first thing a marker checks.
But changing the ending is only a third of it. Spoken Korean also has its own connectives, intensifiers and contractions, and an essay in perfect -ㄴ/는다 that still says 근데 and 진짜 reads as a transcript, not an essay.
1 · The conversion table
| Spoken | Written | Kind |
|---|---|---|
| -아요/어요 · -습니다 | -ㄴ/는다 · -다 | Sentence ending |
| 근데 · 그런데 | 그러나 · 하지만 | But |
| 그래서 | 따라서 · 그러므로 | So |
| 그리고 또 | 또한 | And also |
| 진짜 · 정말 | 실제로 · 매우 | Really |
| 너무 · 되게 | 매우 · 상당히 | Very |
| 좀 | 다소 · 약간 | A bit |
| -거든요 · -잖아요 | -기 때문이다 | Giving a reason |
| 제 생각에는 | (leave it out) | In my opinion |