Grammar
-고 있다
Ongoing action
Form
Verb root + -고 있다 — « to be in the process of ... »
Function
-고 있다 indicates that an action is currently taking place.
Why this form?
The present simple tense -ㄴ/는다 can describe a habit or a current action
-고 있다 clearly highlights how it unfolds.
Examples
보통이라면 간호사나 의사, 심지어 방문객들도 돌아다니고 있어야 한다. — Normally, staff and visitors would be moving around.
지금 책을 읽고 있어요. — I'm reading a book.