습관상 habitual

Habitual is not a homogeneous piece of imperfective.

Rather, it is highly affected by tense.

[습관상은 어떤 단일한 미완망 조각이 아니다. 습관상은 시제에 많이 영향을 받는다]

- The grammaticization of zero, Bybee 1990.

An exclusive present habitual expression tends to be a zero form.

The reason is pragmatic

 : like there is strong overlap between perfective and simple past, 

present is almost same with present imperfective in terms of the meaning 

; Habitual, maybe, is one of the basic aspectual usages of present tense, but not of past. 

; Thus explicit expression of habitual meaning will be more frequent in the past

[현재 습관상 표현은 영형식인 경향이 있다. 현재 시제를 상처럼 쓸 때 기본 용법 중 하나이기 때문이다]

Simple present carries no explicit meaning, 

observing the meaning inhernet to normal social and physical phenomena. 

This meaning consists of habitual occurence and behaviors as well as ongoing states.

[단순 현재는 별도의 의미를 전달하지 않고, 일반 사회/물리 현상에 내재된 의미를 전달

여기에 습관적인 사건발생, 습관적 행위, 진행중인 상태 등이 포함된다]

- Ch. 5. Evolution of Grammar, Bybee et al. 1994