어~ ㅡ Syllable Forms Chosen Conjugation 아~ ㅏ

Adjective examples:
나쁘다 bad
예쁘다 pretty
기쁘다 joyful
크다 big

Verb examples:
쓰다 to write, to use
끄다 to turn off

Conjugation rules when adding -아/어~ ending:

ㅡ is dropped without exception.
The choice between -아~ and -어~ is made in accordance with the vowel in the syllable that comes before the syllable with ㅡ. If the vowel is 아 or 오, the -아~ principle is attached, otherwise the -어~ form is chosen.
Single-syllable stems, for example in 끄다, are affixed by the -어~ form.

If the added ending begins with a syllable different from -아/어~ then the set of regular conjugation rules is applied.

나쁘다 (bad) → 나빠요, 나빴어요, 나빠서, 나빠도 (나쁘니까, 나쁜)
-아~ forms are chosen because of ㅏ in the preceding syllable 나

예쁘다 (pretty) → 예뻐요, 예뻤어요, 예뻐서, 예뻐도 (예쁘니까, 예쁜)

-어~ forms are chosen because there is no ㅏ or ㅗ in 예

기쁘다 (joyful) → 기뻐요, 기뻤어요, 기뻐서, 기뻐도 (기쁘니까, 기쁜)

-어~ forms are chosen because there is no ㅏ or ㅗ in 기

크다 (big) → 커요, 컸어요, 커서, 커도 (크니까, 큰)
쓰다 (to write, use) → 써요, 썼어요, 써서, 써도 (쓰니까, 쓴)
끄다 (to turn off) → 꺼요, 껐어요, 꺼서, 꺼도 (끄니까, 끈)
-어~ forms are chosen because there is no preceding syllable

When forming adverbs by adding -이, ㅡ is dropped.

바쁘다: 바쁘- + -이 → 바삐 (busily, in a hurry)
서글프다 (lonesome): 서글프- + -이 → 서글피 (in a lonesome manner)