Heel simpel, -고 있다 gebruik je wanneer je iets actiefs AAN HET doen bent. 이를 닦고 있어 = Ik ben mijn tanden AAN HET poetsen 영화를 보고 있어 = Ik ben een film AAN HET kijken 뭐 하고 있어? = Wat ben je AAN HET doen? Kan ook in het verleden tijd gebruikt worden. 자고 있었어 = Ik was AAN HET slapen 설거지를 하고 있었어 = Ik was de afwas AAN HET doen Nou deze was eenvoudiger dan jouw vorige vraag. :P

- 고 있다 : 현재형 또는 진행형을 나타낼 때 사용 Korean can express " the present" and " the present progressive " with " - 고 있다 " "- 고 있다" = " - 고 있는 중이다 " ( even though a sentence is 현재 진행형, Koreans commonly express like 현재형. I think that's the reason why you are confused and ask when is the verb form "stem + - 고 있다 ) But English use differently. I'll give you examples. * 현재시제 He reads a newspaper 그는 신문을 보고 있다 * 현재 진행형 He is reading a newspaper 그는 신문을 보고 있다 ( = 그는 신문을 보고 있는 중이다 ) " the present tense " and " the present progressive " are different tense but Korean express in the same way. I'll give you examples. 예) 1 He reads a newspaper 그는 신문을 보고 있다 (the present tense) He is reading a newspaper 그는 신문을 보고 있다 (the present progressive ) (= 그는 신문을 읽고 있는 중이다 ) 2 He eats bread 그는 빵을 먹는다 He is eating bread 그는 빵을 먹는다 (= 그는 빵을 먹고 있는 중이다 )

‘-고 있다’ used when an action is in progress for a long time. It is used with progressive verbs like 자다(to sleep), 공부하다(to study), 먹다(to eat) In such verbs, an action cannot be completed in a short time. On the contrary, verbs like 앉다(to sit), 서다(to stand), 깨지다(to break) which action can be completed in a short time, are not commonly used with ‘-고 있다’

I've made a summery how this would be explained in Dutch/Ik heb een samenvatting gemaakt hoe je dit kan uitleggen in het Nederlands: http://www.koreanwikiproject.com/wiki/TTMIK_niveau_2_les_10