"would have p.p." is conditional III. It is used to talk about imaginary things that didn't happen in the past. Nobody asked me where Sparkle Pony Mountain was, but if they had asked me, I would have told them it was right here. I was so sure that it was right here, that if they asked me to swear an oath promising that it would be here, I would have sworn that it was right here. You can read more about conditionals here: https://www.ef.edu/english-resources/english-grammar/conditional/ Conditionals are usually paired with an "if" clause, but they aren't always.

Hi Taehyun, I am happy to help you with your question. 'Would have' is a modal verb in English. It is used to express your present feelings about a past decision or action. Here are some examples of Would have, could have, should have: # She could have gone to any college she wanted to. # I would have gone to the party, but I was tired. # He should have told the truth about what he saw. Your sentence 'I would have sworn...' is actually incorrect. You need to write 'I could have sworn...'. Here is how you use each of the 3 modal verbs with many more examples: https://www.google.es/amp/s/learningenglish.voanews.com/amp/3391128.html This structure is easier to explain in a lesson with practice. I would be happy to help you further if you book a lesson with me. Good luck with your language journey!

I agree with Chris. The expression "would/could have sworn" is approaching a set idiom.

Other than the grammer a sparkle pony is an attendee at Burning Man who is under-prepared for basic survival but still managed to pack 2 suitcases full of fabulous outfits. https://eplaya.burningman.org/viewtopic.php?t=42029