This pattern uses the nominalizer こと (fact, matter) with the particle に to point to a specific fact as the target of an emotional comment. You put the emotion in the form of a predicate before ことに: for verbs, use the past tense (like 驚いた), and for adjectives and nouns, use the non-past form (like うれしい, 残念な). Then add が to mark it as the subject of the main clause. The whole phrase often comes at the beginning of the sentence, followed by a comma or directly by the main clause. It strongly emphasizes the speaker's feeling, making the sentence more expressive than simply saying 'I was surprised' or 'I'm relieved' in a neutral way. It is common in both spoken and written Japanese, but has a slightly formal, reflective tone. Note: the same spelling ことに can also mean 'every' or 'at intervals of' (e.g., 一時間ことに= every hour), but that is a different grammar point—here, the meaning is emotional emphasis.