I understand that 들다 can be added to some verbs to give emphasis of the direction of the verbs (ex: 날아들다 = fly into), but I got confused with the word 'decrease' (도시의 인구가 계속 줄어들고 있다) What exactly 들다 do when it is attached to a verb? Thank you!
들다 means going "in", "into" or "inward" in the broad sense. 줄어들다 states the process of shrinking more vividly than 줄다, although there is little difference in connotation.
Anything shrinking or dwindling can be thought of as shrinking "inward", even though "down" is the usual preposition to go with shrinking.
A better comparison might be "imploding", where "im" is a variant of "in" and clearly shows the inward nature of it.
The antonym is 늘어나다, where 나다 means going "out" or "outward". You can think of 들다 as "in" in "implode" and "나다" as "ex" in "explode".
Disregarding the more violent implications of the "-plode" words,
- 줄어들다 = 줄다 (decrease, shrink) + -아/어 들다 (move inward) = implode.
- 늘어나다 = 늘다 (increase, expand) + -아/어 나다 (move outward) = explode.
The difference in the nuances is negligible, or I’d say, none. They are interchangeable.
With “-고 있다,” I use both “줄다” and “줄어들다”: “줄고 있다.” and “줄어들고 있다.” for the same meaning. Without it, I usually stick to “줄어들다,” as “줄다” can be confused with other homonyms when conjugated, such as “주다” meaning “to give”: “준다.” and “줄어든다.”