Plain Written Style: -ㄴ/는다

The plain style (한다체) is the neutral written form of Korean: books, newspapers, diaries, essays and signs use it, and close friends use it in casual speech. It carries no politeness ending.

• Verb stem ending in a vowel: -ㄴ다 (가다 → 간다, 공부하다 → 공부한다)
• Verb stem ending in a consonant: -는다 (먹다 → 먹는다, 듣다 → 듣는다)
• Stem ending in ㄹ: drop ㄹ, then add -ㄴ다 (살다 → 산다, 놀다 → 논다)
• Adjectives and 있다/없다 keep the dictionary form: 춥다, 맛있다, 있다
• Past tense: -았/었다 for verbs and adjectives alike (갔다, 먹었다, 했다, 더웠다)
• Nouns: 이다, after a vowel often just -다 (학생이다, 의사다); past -이었다/-였다

This form is also the base of reported speech: 간다 + 고 → 간다고.