Why does 어리다 become 어려요?
When i was studying, i came across "예쁜 여자는 어려요 = The pretty girl is young", I thought, it was suppose to be "예쁜 여자는 어리다" because you're saying the girl is young.

Question

Answer

Because it is present tense in polite form (존댓말).
The second one is the plain form.

Answer

1. 어리다 is a dictionary form and also a plain speech form.
2. 어려요 is a present polite ending form of 어리다.
어리다 is a descriptive verb and its stem is '어리'.  I put a '|' after the stem.

* verb stem + 아요 or 어요 --> present polite ending form

어리| 다 --> 어리+어요 --> 어리어요 --> 어려요 (contracted)

** When the verb stem has 'ㅏ' or 'ㅗ' , '아요' is added.
    When the verb stem has  a vowel other than 'ㅏ' or 'ㅗ', '어요' is added.

*** However, there are exceptions to this vowel rule.
e.g.   아프|다 (to be sick) --> 아파요

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