규칙용언 (Regular predicate)

• Korean predicates (e.g., verbs and adjectives) are regular or irregular. Regular or irregular predicate depends on whether the stem of the predicate is fixed or changes. If the stem does not change or change regularly, which can be explainable with phonological rules(음운 규칙), then such predicates are called the regular predicate. In regular predicates, the stem does not change, but only the ending of the predicates varies regularly. 
  
• But if the stem or ending of word changes cannot be explainable with phonological rules, then such predicates are called the irregular predicate. 
  
• Examples of regular predicates are  "웃다(to laugh)," "먹다(to eat)," "읽다(to read)," "밝다(to be bright)," "맑다(to be clean)."
  

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(1) 웃다(Base form or dictionary form). 웃다 (to laugh): 웃(stem)+다(base ending). 
웃고: 웃(stem) + 고(ending).
그는 TV를 보면서 웃고 있다. (He is laughing while watching TV).
웃지: 웃(stem) + 지(ending).
그는 수업시간이라 큰 소리로 웃지 못했다. (He could not loudly laugh as he was in class).
웃어: 웃(stem) + 어(ending).
사진 찍을 시간이에요, 웃어보아요. (It is time to take a picture; please smile).
웃으니: 웃(stem) + 으니(ending).
엄마가 웃으니 아기도 웃었다. (The baby also laughed as his mom laughed).

(2) 먹다 (Base form or dictionary form). 먹다 (to eat): 먹(stem) + 다(base ending)
먹고: 먹(stem) + 고(ending)
아침밥은 먹고 공부해라. (Study after eating breakfast).
먹지: 먹(stem) + 지(ending)
배부르면 먹지 마라. (If you are full, do not eat).
먹어: 먹(stem) + 어(ending of word) 
이 과일은 맛있으니 먹어 보아라. (As this fruit is delicious, please try it).
먹으니: 먹(stem) + 으니(ending of word).
너무 많이 먹으니 걷기가 힘들었다. (As I overate, it was hard to walk).
As seen above examples, regular predicates (e.g., verbs and adjectives) do not change their stems;
only the ending of word changes to express each situation. For example, the ending "다" is for the completion of the sentence and the ending "고" is for connecting sentences, etc. 

Also, some predicates are considered as regular even though stem changes if phonological rules can explain the change. There are two such cases: '으' 탈락(omission) and 'ㄹ' 탈락(omission). 
In '으' 탈락 ('으' omission), the '으' sound in the stem. For example, '으' in '쓰어' and '으' in '담그' are omitted when endings of words are vowels with the beginning sound of '-아/어' or "-았/-었." In 'ㄹ' omission, when endings of the word begin with (ㄴ, ㅂ, -시, -오), then 'ㄹ' in the stem is omitted.

(1) Example of ‘으’ omission,  쓰다 (write) : 쓰고, 쓰지, 써(쓰+어), 쓰니
쓰다: 쓰(stem)+다(base ending).
쓰고: 쓰(stem)+고(ending).
나는 지금 일기를 쓰고 있다. (I am writing a diary now).
쓰지: 쓰(stem)+지(ending).
글을 쓰지 말고 책을 많이 읽어라. (Do not write, but read many books).
써: Instead of 쓰어, this word becomes 써. 으 in stem 쓰 is omitted as the word's ending is 어 sound.
일기를 써보니 쉽지 않다. (As I wrote a diary, I found that it is not easy).
쓰니: 쓰(stem)+니(ending of the word). 
일기를 매일 쓰니 기분이 좋다. (As I write a diary every day, I feel good).

(2) Example of 'ㄹ' omission.
놀다 (play): 놀(stem)+다(base ending).
놀고: 놀(stem)+고(ending).
그만 놀고 숙제좀 해라. (Stop playing and please do your homework).
놀지: 놀(stem)+지(ending).   
밖에서 놀지 말고 공부좀 해라. (Don't play outside and study).
노는, when the ending word is "는," then "ㄹ" in the stem(놀) is omitted as the ending of the word begin with 'ㄴ.'  That is, the ending (는) of the word (노는) begins with 'ㄴ.' Therefore instead of '놀는,' this word becomes '노는' with the batchim 'ㄹ' is omitted. Batchim is the final sound(consonant) placed below the vowel, the middle sound.
이 추운 겨울에 학교 운동장에서 노는 어린이들을 봐라. (Look at the children playing in the playground during this cold winter). 노니, when the ending of the word is "니," then "ㄹ" in the stem (놀) is omitted as the ending of word 니 begins with 'ㄴ.'  Therefore instead of '놀니,' this word becomes '노니' with the batchim 'ㄹ' is omitted. 
너는 숙제를 안끝내고 또 노니? (Are you playing again without finishing your homework?)

Another example of 'ㄹ' omission.
울다 (verb, base form). 울다(to cry): 울(stem)+다(base ending).
울고: 울(stem)+고(ending). 
그만 울고 저녁 먹어. (Stop crying and eat dinner).
울지: 울(stem)+지(ending).
울지 말고 공부해라. (Do not cry and study).
우는: 우(stem)+는(ending).
When the ending word is "는," then "ㄹ" is omitted from the stem. Therefore instead of '울는,' this word becomes '우는' with the batchim 'ㄹ' is omitted.
우는 아기가 방안에 있다. (There is a crying baby in the room.)
우니, when the ending of the word is "니," then the batchim "ㄹ" is omitted from the stem. Therefore instead of '울니,' this word becomes '우니' with the batchim 'ㄹ' is omitted.
네가 그렇게 슬피 우니 나도 슬프다. (As you cry so sadly, I am also sad).

In the regular predicates, the stem and the ending of the words do not change or change regularly. 
Thus, verbs or adjectives having '으' 탈락(omission) and 'ㄹ' 탈락(omission) can be considered as regular predicates.