Regardless of whether you get the PDF, do this exercise daily:
This is the core technique used in How to Sound Like a Native Korean Speaker.
TTMIK offers Level 9 of their free online curriculum, which covers many "native sounding" patterns. While not the specific PDF, you can download individual lesson PDFs for free from their website.
Go to the official TTMIK store page for this book. They offer a free 20-page sample PDF containing the first two chapters (Contractions and Basic Intonation). Combined with a user-created Anki deck (search "AnkiWeb TTMIK Native Speaker"), you can effectively learn the core 500 natural phrases without buying the full book.
Before the PDF existed, TTMIK released a free audio/video course called "IYAGI: How to Sound Like a Native". Search YouTube for "TTMIK How to Sound Like a Native Korean Speaker playlist" . You get 80% of the audio lessons for free.
Native speakers break rules constantly. The PDF includes sections on 잘못된 표현 (incorrect expressions) that are so common they are accepted, like using 보다 as 봬다.
Korean is a syllable-timed language. Every syllable gets roughly equal time. English speakers stress vowels (SO-mething vs. some-THING). To sound native:
From 대박 (awesome) to 핵인싸 (extreme insider), this section updates you on modern youth slang that no textbook covers.