[Here are] my two bits on the “that’s what I wanna know” discussion. I asked this question years ago, back in the 80s, but never found a satisfactory answer until I read Prof. Sohn’s book, quoted in the attached document. Enjoy!
With D. Bannon’s permission and help from Prof. Sohn’s book, here’s his (as always, very helpful!) explanation:
그것이 알고 싶다!—THAT’S what I want to know!
나는 그것을 알고 싶다/나는 그것이 알고 싶다, does it make ANY sense? It does, actually, and it all depends on the verb. A nominative case particle (이/가) is used in place of usual accusative particle (을/를) to add emphasis. Think of이/가as a verbal italic, as in, “THAT’S what I want to know.” [나는 그것이 알고 싶다] In speech this places the focus on the object of the embedded verb, but the decision for which particle to use is based on the verb itself. The verb dictates if the nominative or accusative must be used or if they are interchangeable, as explained by Ho-Min Sohn:
The desiderative construction with the adjective siphta ‘be wishful, be de-sirable, wish’ is a peculiar type of sensory construction. First, the adjective must be preceded by a clause, which is its object. Second, this object clause is nominalized by the gerundive suffix –ko. Third, when the clause before –ko siphta is transitive, the object of the embedded verb may be marked with either a nominative or an accusative particle. When siph-e hata occurs, the object is always in the accusative case.
na nun kheyik i/ul mek-ko (ga/lul) siph-ta
I TC cake NM/AC eat-NOM NM/AC wish-DC
‘I want to eat cake.’
Mia nun kyeyik i/ul mek-ko (ga/lul) siph-e ha-n-ta
Mia TC cake NM/AC eat-NOM NM/AC wish-INF do-IN-DC
‘Mia wants to eat cake.’
Simply put, if the verb is desiderative, as with –고 싶다, a nominative case particle may be used to add emphasis. Again from Sohn:
The desiderative adjective siphta ‘be desirable, wish’ is a special transitive sensory adjective. It is a bound adjective and is used only when preceded by a verb clause that ends in the nominalizer suffix –ko.
na nun ku chinkwu ka po-ko siph-e
I TC the friend NM see-to wishful-INT
‘I wish to see that friend.’
이/가 plays an essential role in spoken language, bringing the focus of a given sentence directly to the most important point of the speech—which may or may not be the subject of the sentence. As Sohn explains:
The accusative particle alternates with the nominative particle in causative sentences. . . . Desiderative sentences show similar alternation.
hyeng un tampay lul/ka phiwu-ko siph-ess-e-yo
brother TC cigarette AC/NM smoke-NOM wish-PST-POL
‘My older brother wanted to smoke.’
In desiderative sentences, the accusative-marked nominal is associated with the transitive verb (e.g., phiwuta ‘smoke’), whereas the nominative-marked nominal is related to the emotive adjective siphta ‘wish’, as in hyeng un [tampay lul phiwu-ko] siph-ess-ta and hyeng un tampay ka [phiwu-ko] siph-ess-ta, respectively.
If the emphasis is on my own curiousity, I would say, “That’s what I want to know.” 나는 그것을 알고 싶다. However, in colloquial usage, the desiderative auxiliary verb indicates “that the speaker or subject wishes for the action or state of the main verb to happen or come about,” as explained Ihm, Ho Bin, Hong, Kyung Pyo and Chang, Suk In. The emphasis rests on the object to be known, requiring the nominative case particle to emphasize this point. So why the이/가 nominative case particle? 그것이 알고 싶다!
Im, Ho Bin, et al. Korean Grammar for International Learners: New Edition. Yonsei University Press (2001): 354. Translated into English by Ross King.
Sohn, Ho-Min. Cambridge Language Surveys: The Korean Language. Cambridge University Press (2001): xix-xx, 287, 331, 384.
Key to Sohn’s abbreviations:
NM Nominative case particle
TC Topic-contrast particle
INT Intimate speech level or suffice
AC Accusative particle
NOM Nominalizer suffic
PST Past tense and perfect aspect suffix
POL Polite speech level suffix or particle
INF Infinitive suffix
DC Declarative sentence-type suffix