Describing things

English needs 'is' plus an adjective: 'it is small'. Korean folds the 'is' into the word itself, so 작다 means 'to be small' and 작아요 is a complete sentence. Because they are verbs, describing words do everything verbs do: 안 작아요 'it is not small' with day 21's 안, 작았어요 'it was small' with day 20's past. The one difference is that they describe rather than act, so the thing described takes 이/가, as 커피가 좋아요 did on day 23.

ㅡ is the weakest vowel in Korean, a neutral sound that is often there only to hold a syllable together. When another vowel arrives, ㅡ drops out and the consonant attaches to the new vowel. With ㅡ gone, the stem's last vowel can no longer decide between 아요 and 어요, so Korean looks one syllable further back: ㅏ or ㅗ there gives 아요, anything else gives 어요. A one-syllable stem like 크 has nothing to look back at and takes the default, 어요.

These stems once ended in a soft consonant, somewhere between b and w, that Korean later lost. Before a consonant it hardened into ㅂ, which is why the dictionary form is 어렵다; before a vowel it softened into 우, which is why you hear 어려워요. The change only happens before a vowel, so an ending that starts with a consonant leaves the ㅂ alone. Most describing verbs that end in ㅂ follow this pattern, but some action verbs such as 입다 'to wear' are regular (입어요), and 돕다 'to help' gives 도와 rather than 도워, which you will need tomorrow in 도와주세요.