You can find a complete explanation for this, on the link below: www.talktomeinkorean.com/lessons/level-5-lesson-16/ :)

This ending is only used towards someone who is NOT older than you or are on close terms with. This style doesn't convey much politeness compared to the -요/-습니다 endings. You use -(ㄴ/는)다 for VERBS; and if you want to learn what is used for ADJECTIVES, you have to use Verb stem + 다. - Verb stems ending with a vowel + ㄴ다 자다 → 잔다 - Verb stems ending with last consonant + 는다 곱다 → 곱는다 If a verb ends with ㄹ, you drop it and add ㄴ다 팔다 → 판다 Keep in mind that there are both verbs and adjectives among ∼하다 predicates. - 하다 verbs take ∼는/ㄴ다 for their non-past forms, e.g., 공부한다, 하숙한다, 한국말을 잘 한다, On the other hand, ∼하다 adjectives simply takes ∼다, as in 친절하다, 깨끗하다 USAGES: A. (Towards someone younger than you, is in close terms with, towards a child, sibling, etc.) 1) Showing your impression or reaction when talking about a present action. - - - EX: - - - a. 전화 온다 2) Talking about a present action, situation or a regular activity - - - EX - - - a. 이번에 하숙집으로 옮기려고 한다 b. 나는 저녁 6시에 집에 돌아온다 B. Mainly used in writings such as prose, expository writings, newspaper articles, journals, academic writings, etc. This is a very common way of describing a series of actions whenever a very neutral and narrative voice is required.

I'm a beginner myself in Korean. but i think you use ㄴ다 when writing a verb in the present tense which doesn't have a consonant as the last character. For example: 오다= to come, in the present written form it's written as 온다. or another example is 가다= to go>> 간다- going You'd use 는다 if the verb already has an ending with a consonant, for example: 먹다- to eat>> 먹는다= eating. This is from memory, but I hope it helped.