1. In the sense that the meaning is "to have his strong assurance shivered at a blow might one day be agony to him", yes, but native speakers would not go through such a mental process, and (probably) would not consider "it" to refer to anything at all. Patterns like this, in which a dummy or anticipatory "it" is put to the front, are a common feature of English, as you have found.
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park sang joon4. I'd also like to know if the subject of "take up" is the "the newspaper."No; 'him'.
park sang joon4. I'd also like to know if the subject of "take up" is the "the newspaper."No, "take up" is an infinitive; the person doing the action "take up" is the same person as "him". The pattern is "to know someone do something", meaning to be aware or have observed that someone acted in a certain way.
park sang joonThen I was wondering if this usage of "know" is idiomatic in present English.'To know someone to do something'? Yes. I have been known to give the wrong answers to questions sometimes.
park sang joonI meant "know + object + a bare infinitive" by the usage as in "I have known him take up his stick"Yes, this pattern is still used in modern English. I think it is mostly used in the present perfect or past perfect tenses.