In written Korean for diaries and the like, we aren't always sure who the reader might be and therefore it's awkward to write using endings such as 요 and 습니다. So therefore we use the written form.
Present tense verb + (느)ㄴ다
eg. 하다 -> 한다
eg. 먹다 -> 먹는다
Past tense verb + 았/었다
eg. 하다 -> 했다
eg. 먹다 -> 먹었다
Future tense verb + (으)ㄹ 것이다
eg. 하다 -> 할 것이다
eg. 먹다 -> 먹을 것이다
Adverb (dictionary form)
eg. 예쁘다 (no change)
eg. 귀엽다 (no change)