Return to Answer

• All of the sentences in your question sound good and natural to me.
• Grammatically, both are identical.
• Not this one, but there are some cases where fuller forms are not idiomatic and sounds more awkward. E.g. “너 심부름 가니?” sounds much better than “너 심부름을 가니?”. The exact reasons and rules ⸺ I don’t know.
• Those without particles/markers are more common and seemingly preferred by native speakers.
• It is not that you can drop any particles/markers and make sense. The exact rules for what and when you can omit, if any, are complicated. Basically, if omitting a marker doesn’t open it up to misinterpretation of the class (e.g. a subject might be misinterpreted as an object without a subject marker), it’s usually okay to drop it.
  • Types of particles you could omit (couldn’t find documented rules though):
    • 주격 조사_{Agentive particle (subject marker)} “이”/“가”
    • 목적격 조사_{Accusative particle (object marker)} “을”/“를”
    • 속격 조사_{Genitive particle “’s”} “의”
    • 향격 조사_{Lative particle “to”} “에”/“로”
    • and possibly more.

• All of the sentences in your question sound good and natural to me.
• Grammatically, both are identical.
• Not this one, but there are some cases where fuller forms are not idiomatic and sounds more awkward. E.g. “너 심부름 가니?” sounds much better than “너 심부름을 가니?”. The exact reasons and rules ⸺ I don’t know.
• Those without particles/markers are more common and seemingly preferred by native speakers.
• It is not that you can drop any particles/markers and make sense. The exact rules for what and when you can omit, if any, are complicated. Basically, if omitting a marker doesn’t open it up to misinterpretation of the class (e.g. a subject might be misinterpreted as an object without a subject marker), it’s usually okay to drop.
  • Types of particles you could omit (couldn’t find documented rules though):
    • 주격 조사_{Agentive particle (subject marker)} “이”/“가”
    • 목적격 조사_{Accusative particle (object marker)} “을”/“를”
    • 속격 조사_{Genitive particle “’s”} “의”
    • 향격 조사_{Lative particle “to”} “에”/“로”
    • and possibly more.

It’s about omitting postpositions/markers.

• All of the sentences in your question sound good and natural to me.
• Grammatically, both are identical.
• Not this one, but there are some cases where fuller forms are not idiomatic and sounds more awkward. E.g. “너 심부름 가니?” sounds much better than “너 심부름을 가니?”. The exact reasons and rules ⸺ I don’t know.
• Those without postpositions (markers) are more common and seemingly preferred by native speakers.
• It is not that you can drop any postpositions and make sense. The exact rules for what and when you can omit, if any, are complicated. Basically, if omitting a marker doesn’t open it up to misinterpretation of the class (e.g. a subject might be misinterpreted as an object without a subject marker), it’s usually okay to drop it.
  • Types of postpositions you could omit (couldn’t find documented rules though):
    • 주격 조사_{Agentive postposition (subject marker)} “이”/“가”
    • 목적격 조사_{Accusative postposition (object marker)} “을”/“를”
    • 속격 조사_{Genitive postposition “’s”} “의”
    • 향격 조사_{Lative postpostion “to”} “에”/“로”
    • and possibly more.

Omitting agentive postpositions (subject markers)

TL; DR: if a subject is specific and always interpreted as a subject (“never misinterpreted as an object”) even without a subject marker, you can drop the marker.

‘너 심부름 가니?/저 개 아주 사납다/OOO의 소설 재미있더라.’와 같이, 대명사나 그 지시 대상이 특정적인, 한정적 표현의 명사가 주어 자리에 올 때 주어가 주격 조사 없이 쓰일 수 있다. When a subject is a pronoun or a specific noun, you can omit the agentive postposition (subject marker).

• ‘너 심부름 가니?’ “Are you going on an errand?”
• ‘저 개 아주 사납다.’ “That dog is so aggressive.”
• ‘……의 소설 재미있더라.’ “……’s novel was interesting to read.”

하지만 ‘개 아주 사납구나./소설 재미있더라.’와 같이, 주어 자리에 오는 명사가 비한정적일 때에 주격 조사가 쓰이지 않으면 비문(非文)이 된다. If you don’t put an agentive postposition (subject marker) on a subject that is not specific, the sentence becomes ungrammatical.

• ‘개 아주 사납구나.’
• ‘소설 재미있더라.’

또한 ‘그 여자 무얼 주려고 하는데.’와 같이 주격 조사가 없이 쓰인 체언이 주어 이외의 것(‘그 여자가’ 혹은 ‘그 여자한테’)으로 혼동될 우려가 있을 때는 주어 자리에 오는 체언이 비록 한정적이라도 주격 조사가 쓰여야 한다. If a subject without an agentive postposition (subject marker) could be misinterpreted as a non-subject, you have to keep the agentive postposition (subject marker), whether the subject is specific or not.

• The ambiguous sentence ‘그 여자 무얼 주려고 하는데.’ (“The girl; about to give something.”) can be interpreted as:
  1. ‘그 여자(가)_{[subject “She”]} (누구에게)_{[implied object “to someone”]} 무얼 주려고 하는데.’ “She(’s) about to give something (to someone).”
  2. ‘(내가)_{[implied subject “I”]} 그 여자(한테)_{[object “to her”]} 무얼 주려고 하는데.’ “(I’m) about to give something (to) her.”

⸻ Gisim Nam, ‘현대 국어 통사론’ (“The syntax of modern Korean”), published by 태학사 (Taehaksa), in 2001.

It’s about omitting particles/markers.

• All of the sentences in your question sound good and natural to me.
• Grammatically, both are identical.
• Not this one, but there are some cases where fuller forms are not idiomatic and sounds more awkward. E.g. “너 심부름 가니?” sounds much better than “너 심부름을 가니?”. The exact reasons and rules ⸺ I don’t know.
• Those without particles/markers are more common and seemingly preferred by native speakers.
• It is not that you can drop any particles/markers and make sense. The exact rules for what and when you can omit, if any, are complicated. Basically, if omitting a marker doesn’t open it up to misinterpretation of the class (e.g. a subject might be misinterpreted as an object without a subject marker), it’s usually okay to drop it.
  • Types of particles you could omit (couldn’t find documented rules though):
    • 주격 조사_{Agentive particle (subject marker)} “이”/“가”
    • 목적격 조사_{Accusative particle (object marker)} “을”/“를”
    • 속격 조사_{Genitive particle “’s”} “의”
    • 향격 조사_{Lative particle “to”} “에”/“로”
    • and possibly more.

Omitting agentive particles (subject markers)

TL; DR: if a subject is specific and always interpreted as a subject (“never misinterpreted as an object”) even without a subject marker, you can drop the marker.

‘너 심부름 가니?/저 개 아주 사납다/OOO의 소설 재미있더라.’와 같이, 대명사나 그 지시 대상이 특정적인, 한정적 표현의 명사가 주어 자리에 올 때 주어가 주격 조사 없이 쓰일 수 있다. When a subject is a pronoun or a specific noun, you can omit the agentive particle (subject marker).

• ‘너 심부름 가니?’ “Are you going on an errand?”
• ‘저 개 아주 사납다.’ “That dog is so aggressive.”
• ‘……의 소설 재미있더라.’ “……’s novel was interesting to read.”

하지만 ‘개 아주 사납구나./소설 재미있더라.’와 같이, 주어 자리에 오는 명사가 비한정적일 때에 주격 조사가 쓰이지 않으면 비문(非文)이 된다. If you don’t put an agentive particle (subject marker) on a subject that is not specific, the sentence becomes ungrammatical.

• ‘개 아주 사납구나.’
• ‘소설 재미있더라.’

또한 ‘그 여자 무얼 주려고 하는데.’와 같이 주격 조사가 없이 쓰인 체언이 주어 이외의 것(‘그 여자가’ 혹은 ‘그 여자한테’)으로 혼동될 우려가 있을 때는 주어 자리에 오는 체언이 비록 한정적이라도 주격 조사가 쓰여야 한다. If a subject without an agentive particle (subject marker) could be misinterpreted as a non-subject, you have to keep the agentive particle (subject marker), whether the subject is specific or not.

• The ambiguous sentence ‘그 여자 무얼 주려고 하는데.’ (“The girl; about to give something.”) can be interpreted as:
  1. ‘그 여자(가)_{[subject “She”]} (누구에게)_{[implied object “to someone”]} 무얼 주려고 하는데.’ “She(’s) about to give something (to someone).”
  2. ‘(내가)_{[implied subject “I”]} 그 여자(한테)_{[object “to her”]} 무얼 주려고 하는데.’ “(I’m) about to give something (to) her.”

⸻ Gisim Nam, ‘현대 국어 통사론’ (“The syntax of modern Korean”), published by 태학사 (Taehaksa), in 2001.

• All of the sentences sound good and natural to me.
• Grammatically, both are identical. Not this one, but there are some cases where fuller forms are not idiomatic and sounds more awkward. E.g. “너 심부름 가니?” sounds much better than “너 심부름을 가니?”. The exact reasons and rules ⸺ I don’t know.
• Although those without postpositions are more common and seemingly preferred by native speakers, it is not that you can drop any postpositions and make sense. From a related paper:

Some other postpositions you could omit (couldn’t find documented rules though):

• 목적격 조사_{Accusative postposition (object marker)} “을”/“를”
• 속격 조사_{Genitive postposition “’s”} “의”
• 향격 조사_{Lative postpostion “to”} “에”/“로”
• and possibly more.

• All of the sentences in your question sound good and natural to me.
• Grammatically, both are identical. 
• Not this one, but there are some cases where fuller forms are not idiomatic and sounds more awkward. E.g. “너 심부름 가니?” sounds much better than “너 심부름을 가니?”. The exact reasons and rules ⸺ I don’t know.
• Those without postpositions (markers) are more common and seemingly preferred by native speakers.
• It is not that you can drop any postpositions and make sense. The exact rules for what and when you can omit, if any, are complicated. Basically, if omitting a marker doesn’t open it up to misinterpretation of the class (e.g. a subject might be misinterpreted as an object without a subject marker), it’s usually okay to drop it.
  • Types of postpositions you could omit (couldn’t find documented rules though):
    • 주격 조사_{Agentive postposition (subject marker)} “이”/“가”
    • 목적격 조사_{Accusative postposition (object marker)} “을”/“를”
    • 속격 조사_{Genitive postposition “’s”} “의”
    • 향격 조사_{Lative postpostion “to”} “에”/“로”
    • and possibly more.

A related paper (See the comments.)