Based on the level of respect or formality, you can say "I didn't do this on purpose" in Korean in 3 ways:

• Informal: na-neun ilbureo i-geot-eul haji anatseo (나는 일부러 이것을 하지 않았어)
• Formal: jeo-neun ilbureo i-geot-eul haji anatseoyo (저는 일부러 이것을 하지 않았어요), jeo-neun ilbureo i-geot-eul haji anatseupnida (저는 일부러 이것을 하지 않았습니다)
• Very Formal: jeo-neun ilbureo i-geot-eul haji anatseupnida (저는 일부러 이것을 하지 않았습니다)

- I: na-neun (나는), jeo-neun (저는)
- to do: hada (하다)
- this (objective): i-geot-eul (이것을), i-geol (이걸)
- purpose: mokjeok (목적)
- on purpose: ilbureo (일부러)

Please refer to How to pronounce Korean Alphabet for the English pronunciation in this answer.