Breakdown of Čim sam pročitao knjigu, on je otišao.
biti
to be (sam, si, je)
on
he
knjiga
book
otići
to leave, to go away
Questions & Answers about Čim sam pročitao knjigu, on je otišao.
Why do we use pročitao instead of čitao for "read"?
pročitao is a perfective verb, which focuses on a completely finished action. The sentence means "As soon as I finished reading the book". If you used the imperfective čitao, it would mean "As soon as I was reading", which does not fit the logic of the sentence.
Why is it otišao rather than išao?
otišao (from the verb otići) is the perfective form for "left" or "went away". We use it here because the action of leaving was a single, completed event. išao (from ići) is imperfective and means "was going" or "went" in a continuous sense.
Does the word čim (as soon as) usually require the perfective aspect?
Yes, almost always! Because čim points to the exact moment an action finishes so that a new one can begin, it pairs naturally with perfective verbs like pročitao to highlight that the first action is 100% complete.
Could we leave out the word on (he) in the second part of the sentence?
Yes, but including on is very helpful here. The first part of the sentence is about "I" (sam pročitao). Using on clearly marks the switch to a new subject. If you dropped it, the word order would change to otišao je, because the tiny helper verb je cannot stand at the beginning of its clause after the comma.