All verbs will show up in the dictionary with 다 at the end of them. This is referred to as the "dictionary form". The dictionary form must be conjugated for use outside the dictionary (but stand by, cause DVs have the easiest conjugation possible).
AV (Action Verbs 동작동사)
Action verbs must be conjugated, at the very least, with ㄴ/는 as follows (of course other conjugations 먹어, 먹어요, 먹습니다 still exist but aren't currently relevant to understanding this concept):
먹다 => 먹는다! (Subject eat/eats!)
하다 => 한다! (Subject do/does!)
읽다 => 읽는다! (Subject read/reads!)
DV (Descriptive Verbs 형용사)
Descriptive verbs can be used directly in their dictionary form:
예쁘다 => 예쁘다! (Subject is/am/are pretty!)
좋다 => 좋다! (Subject is/am/are good!)
쉽다 => 쉽다! (Subject is/am/are easy!)
This is not to say that other conjugations of DV are impossible: 예뻐, 예뻐요, 예쁩니다, 좋아, 좋아요, 좋습니다 and the like are perfectly fine, too. It seems like your question is primarily on the 다 which hopefully is now clear.
Now let me point out something else about what happens when you drop the 다. Verb conjugations may require 아/어 as they conjugate...so how do you know if it's 아 or 어? I'm glad you asked.
- If a verb ends in 아 or 어, do not add 아/어
가다 => 가! or 가요! (go!)
- If a verb's final syllable contains a dark vowel sound add 어
치다 => 쳐! (note the 이 combines with the 어) (kick!)
읽다 => 읽어! or 읽어요! (read!)
- If a verb's final syllable contains a light vowel sound add 아
좋다 => 좋아! or 좋아요! (good!)
아프다 => 아파! or 아파요! (ow!)
And then of course 하다 is unfortunately irregular for adding 아/어:
하다 (adds 어 and when 아 + 어 combine they make 애) => 해! or 해요! (do it!)
And there are many other irregular verbs, for instance "ㅂ verbs" and "르 verbs".
In ㅂ verbs the ㅂ becomes 우 which is dark so you add 어:
쉽다 => (쉬 + 우 + 어) 숴워! or 쉬워요! (easy!)
In 르 verbs the ㅡ (으) is dropped and an extra ㄹ goes into the previous syllable and since ㅡ (으) is considered (in this case) light we add 아:
빠르다 => (빠 + ㄹ + ㄹ + 아) 빨라! or 빨라요! (fast!)
"But I heard someone say 아퍼, what gives?"
It's not uncommon for native Koreans to use a dark sound where a light sound should have been. While 아파 is the 표준어 (standard/correct language), 아퍼 is quite common, ain't it?