In normative grammar (school grammar),
-서 is a subjective case marker dedicated to cardinal number subjects. To that end, the sentence "아이가 혼자서 잘 놉니다" is a double-subject phrase, and some might find it a bit unnatural, if not ungrammatical.
- 아이 혼자서 잘 놉니다. ~= 아이가 혼자 잘 놉니다.
- ?아이가 혼자서 잘 놉니다.
Note that the above structure is different from the following
, in that this is a subordinate sentence with two subjects and two predicates corresponding respectively.
However, in the real world, one might find lots of usage of this type of "double-subject" sentence. You'll find many, for example, when you try googling "가 둘이서".
Studies proposed different interpretations of the grammatical function of
-서/이서, while attempting to explain such adverbial usages of number +
-서/이서 phrases. The majority of linguists agree that it is more than a subjective case marker, and some suggest that
-서/이서 is not a subjective case marker at all, but an adverbial derivational suffix. For example, here's a recent survey paper: (paywalled) https://www.kci.go.kr/kciportal/ci/sereArticleSearch/ciSereArtiView.kci?sereArticleSearchBean.artiId=ART002419075
Back to original questions,
wouldn't that sentence carry the same meaning? is there an additional nuance I'm missing?
Yes, they carry pretty much the same meaning, and I don't think they make any significant difference in nuance. But a school grammar enthusiast might disagree with me. Compared to
혼자 as an adverb which is more neutrally describing "the state of being alone", though, there is a minor shift in the nuance to "actively performing an action as a single person/entity" when
-서 is added. This doesn't make a big difference in the original sentence where a kid is playing, but, for example,
- 혼자 죽었어요. In a group, only one died.
- 혼자서 죽었어요. Committed suicide, or did something very stupid or risky that led to the death of the actor.
what's the grammar behind this 서 particle?
This is a longstanding research topic in the field, and unfortunately, there's no clear answer to this question.