Like the title says, can anyone give an explanation on the difference between nominal and pronominal cases?
Based on the summary on this page, it appears to be terminology completely orthogonal to the function of the case used to distinguish which parts of speech the case applies to.
If I understood the distinction correctly, you could, therefore, talk about the nominal accusative case, the pronominal accusative case, or even adjectival accusative case if you were dealing with a language that marks case only on some parts of speech, but not others. In that respect, you could, I suppose, describe English as having pronominal nominative, accusative, dative and genitive cases as vestiges of a case system that has disappeared for nouns and adjectives.
I imagine that you came across it in a conlanging context because a fair number of conlangers deliberately set out to make their language different from what is found in natural languages, so even if asymmetric case-marking is rare in reality, a conlanger might decide to create a language where case is marked on nouns but not on pronouns, or vice-versa, depending on whatever rule struck their fancy.
The nominative case refers to how the subject of a verb in a given language will change form simply by being the subject. Present day English does not change the forms of most nouns simply because they are the subject. For example,
- Sally likes John.
- John likes Sally.
Sally did not change form despite going from the subject position in (1) to the direct object position in (2). Wikipedia has a great example of Japanese displaying the use of nominative case to clearly mark the subject of a verb.
Kabin-ga(S) kowareta vase-NOM broke ’A vase broke’ Watashi-wa(S) kabin-wo(O) kowashita I-NOM vase-ACC broke ’I broke the vase’
The subjects are marked with (S), the objects marked with (O) in the first line. In the second line, NOM stands for the nominative case and ACC stands for the accusative (or direct object) case. What these examples show is that
-wa mark the nominative case of the subject of a verb. So, a Japanese speaker could technically scramble the word order and just by hearing
Kabin-ga know that
Kabin-ga is the subject of some verb.
The pronominal case is similar, but specifically for how pronouns change form. English has pronominal case, for example
- I like Susy.
- Susy likes me.
I has become
me simply by going from subject to direct object position.
One difference between pronominal and nominative case, then, is that pronominal case refers to the changing of the form of a pronoun depending on its position in a sentence while the nominative case refers to the changing of the form of words in specifically the subject position.