It just seems to be a missing "know". There's nothing in the rest of the sentence it could refer to, nor would it fit with the following clause introduced by "when". Semantically it has to be "know".
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park sang joonI'd like to know whether that omission is acceptable.No, it's just a typing error.
khoff It would make more sense for you to try to learn from professional writing that has been edited, not random individuals' careless, informal comments.Beyond a certain stage of learning, I do not especially agree with this. If you want to use English for real, then you need to be able to cope with badly written or typo-ridden language too, especially nowa
khoffI just don't think it makes sense to try to analyze whether something is a past participle or a predicate adjective or whatever when it's actually just a mistake.Indeed.