Hi Jeffery,
If Greek is defined by the English grammar needed to translate it - as some grammarians still do - then the internal subject of ἦν could be used as the (pseudo)subject and ἄνθρωπος, Νικόδημος, and ἄρχων would be the predicates. This happens in other other uses of the verb
to be too. Since the introduction of pseudo-subjects is a requirement of English, the eternal, ἐγώ εἰμι "I am" statements of Jesus (e.g. Jn.13:19) seem ungrammatical, while the ordinary statements where ἐγώ εἰμι may be translated as "it's me" (e.g. Mt.14:27) are natural enough to escape notice. As an other example of how the needs of the English language in this regard are quite demanding, have a look at this sign from Kunming airport:

- mmexport1508402506239.jpg (329.3 KiB) Viewed 3917 times
It was intended to help passengers understand where they are in the airport, but if the simple sentence were to be kept in translation, something like, "This is where you are" might prompt readers to look for other information on the sign, rather than looking at their feet.
If Greek grammar is defined in terms of Greek grammar, then ἄνθρωπος, Νικόδημος, and ἄρχων are the explicated subjects of the internal subject of ἦν.
In most cases, translational grammar and analytical or systemic grammar coincide, but not always - this is one of those non-coincidental cases. What you have brought up in your question is an example of where translational grammar interferes with or alters our understanding of analytical or systemic Greek grammar. An example of where analytical or systemic grammar affects translational grammar is when a beginner applying their understanding of Greek to the translation into English and translates λέγει αὐτοῖς ὁ Ἰησοῦς as "Jesus he says to them". The inclusion of tge spurious "he" demonstrates a knowledge of analytical or systemic Greek grammar, which is good, but that knowledge is inappropriately applied in a translation task, where a translational grammar should be used.
In brief, you have correctly identified that there are two competing and yet complimentary Greek grammars that we need to master. In college or university exams on Greek, both types of knowledge are usually tested - a typical question is "translate this passage and comment on the grammar of the underlined words and phrases".
You are on the right track. Balanced learning means learning to understand Greek in itself, and it also means developing the skills needed to render your new found knowledge of Greek texts into a language that others can understand and appreciate.