The beginner stage of starting to learn a field or skill
When to use
Starting new field, Basic learning, Beginner-level class
The nuance
Use 입문 when someone is starting with little or no prior knowledge or experience in the field.
입문 (ip-mun, ipmun) · 중급 (jung-geup, junggeup) · 고급 (go-geup, gogeup)
입문, 중급, and 고급 represent stages of skill or learning levels: 입문 is beginner, 중급 is intermediate, and 고급 is advanced expertise.

One lane per word — no nested boxes, just the gist.
The beginner stage of starting to learn a field or skill
When to use
Starting new field, Basic learning, Beginner-level class
The nuance
Use 입문 when someone is starting with little or no prior knowledge or experience in the field.
An intermediate stage with some experience and knowledge beyond basics
When to use
Has basic knowledge, Some practical experience, Intermediate difficulty tasks
The nuance
중급 is suitable when someone understands basic concepts and has practical experience, learning more complex material.
An advanced level with professional and in-depth knowledge and skills
When to use
Expert level, Complex problem solving, Advanced study and research
The nuance
Use 고급 when someone has professional expertise and can handle complex problems or creative applications.
이 강의는 입문자를 위한 기초 문법을 다룹니다.
i gang-ui-neun ip-mun-ja-reul wi-han gi-cho mun-beop-eul da-rup-ni-da., i gang ui neun ip mun ja reul wi han gi cho mun beop eul da rup ni da.
The sentence suits beginners, making it natural to use 입문 here.
중급 수업에서는 기본 문법만 다룹니다.
jung-geup su-eop-e-seo-neun gi-bon mun-beop-man da-rup-ni-da., jung geup su eop e seo neun gi bon mun beop man da rup ni da.
중급 implies beyond basics, so 'only basic grammar' is inappropriate; it should include applied grammar.
고급 학습자는 복잡한 문장 구조를 이해할 수 있습니다.
go-geup hak-seup-ja-neun bok-jap-han mun-jang gu-jo-reul i-hae-hal su iss-seup-ni-da., go geup hak seup ja neun bok jap han mun jang gu jo reul i hae hal su iss seup ni da.
Advanced learners can understand complex structures, so the sentence is appropriate.
입문 단계에서는 전문적인 연구를 진행합니다.
ip-mun dan-gye-e-seo-neun jeon-mun-jeok-in yeon-gu-reul jin-haeng-hap-ni-da., ip mun dan gye e seo neun jeon mun jeok in yeon gu reul jin haeng hap ni da.
Beginner stage is for basics, so conducting professional research is unsuitable here.
중급 실력자는 실무에 어느 정도 참여할 수 있습니다.
jung-geup sil-ryeok-ja-neun sil-mu-e eo-neu jeong-do cham-yeo-hal su iss-seup-ni-da., jung geup sil ryeok ja neun sil mu e eo neu jeong do cham yeo hal su iss seup ni da.
Intermediate level can participate in practical work to some extent, making the sentence natural.
고급 과정은 초보자를 위한 입문 과정입니다.
go-geup gwa-jeong-eun cho-bo-ja-reul wi-han ip-mun gwa-jeong-ip-ni-da., go geup gwa jeong eun cho bo ja reul wi han ip mun gwa jeong ip ni da.
Advanced courses are not for beginners, so this sentence is contradictory.
입문용 교재는 쉽게 쓰여 있습니다.
ip-mun-yong gyo-jae-neun swip-ge sseu-yeo iss-seup-ni-da., ip mun yong gyo jae neun swip ge sseu yeo iss seup ni da.
입문용 refers to materials for beginners, so this usage is appropriate.
중급자는 기본 개념을 전혀 모릅니다.
jung-geup-ja-neun gi-bon gae-nyeom-eul jeon-hyeo mo-reup-ni-da., jung geup ja neun gi bon gae nyeom eul jeon hyeo mo reup ni da.
Intermediate learners must know basic concepts, making this sentence incorrect.
Which level means a person just starting to learn?
Which level requires complex problem-solving skills?
Which level knows basic concepts and has some practical experience?