Classical Hebrew: נהר פלגיו ישמחו

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יֶהֱמ֣וּ יֶחְמְר֣וּ מֵימָ֑יו
  יִ֥רְעֲשֽׁוּ הָרִ֖ים בְּגַאֲוָת֣וֹ סֶֽלָה׃
נָהָ֗ר פְּלָגָ֗יו יְשַׂמְּח֥וּ עִיר־אֱלֹהִ֑ים
  קְ֝דֹ֗שׁ מִשְׁכְּנֵ֥י עֶלְיֽוֹן׃

(from Psalm 46)

Is נָהָ֗ר the subject of the sentence? Why is it translated "There is a river"?

  • It's better to think of the subject of verb rather than of a sentence. If you mean the subject of the verb ישמחו, then it would seem the subject is פלגיו.
    The subject of what?

    As I just said, it's better to think of the subject of a verb rather than of a sentence.
    Before asking what its role is, first ask what this means. The answer may be obvious after that.
    In Hebrew syntax this type of sentence is called משפט ייחוד (I don't know the English term). In such a sentence one word (or phrase) is moved to the beginning of the sentence, and another word/preposition/suffix referring to it is inserted in the original place of the moved word. The part moved to the beginning (called חלק הייחוד) is usually an object but rarely the subject of the sentence.

    The verse in question is equivalent to פלגי נהר ישמחו עיר אלוהים; here נהר was moved to the beginning and replaced by the possessive suffix ־ו, resulting in נהר פלגיו ישמחו עיר אלוהים. Thus, the subject is פלגי נהר.

    There are other examples of this structure in the Bible:
    ירושלים הרים סביב לה (Psalms 125:2)
    החכם עיניו בראשו (Ecclesiastes 2:14)

    By the way, משפטי ייחוד exist in modern Hebrew too:
    תקנות אלה כוחן יפה גם לתחומים אחרים
    הנבחרת הלאומית, מקומה באולימפיאדה מובטח
    חוקי המדינה - יש לציית להם
    Without consulting the scholarship, my guess is that the indefinite form of נָהָר has been taken as indicating that it is predicative. My solution for that is that the noun is well-attested in Ugaritic as a quasi-proper name, i.e. quasi divine name. I propose the somewhat whimsical translation '(as for) Mr. River' (i.e. a quasi-personified river), which creates a strong similarity with amikama's example with fronting of Jerusalem. Observe:

    יְֽרוּשָׁלַ֗͏ִם הָרִים֮ סָבִ֢יב לָ֥֫הּ
      וַ֭יהֹוָה סָבִ֣יב לְעַמּ֑וֹ
      מֵ֝עַתָּ֗ה וְעַד־עוֹלָֽם׃
    (תהלים קכה ב)
    You are overthinking it. It's perfectly normal for an indefinite nouns to be used as definite in poetic passages of the Bible.
    Could you please give me an example of one being used as the subject of a nominal sentence?
    I can, but it would have no relevance here. There is no nominal sentence in the words we are discussing.
    נָהָ֗ר פְּלָגָ֗יו יְשַׂמְּח֥וּ עִיר־אֱלֹהִ֑ים

    Isn’t this nominal? How else would you account for the fact that it begins with a noun? Note that the verb is in the plural.
    That’s the implication of the analysis as a quasi-proper noun.
    What I was saying is there is no need to come up with a convoluted theory where it is a "quasi-proper noun" when the simple explanation is much more likely and fits perfectly well.
    It's a casus pendens of the genitive type, because the pronominal suffix on פְּלָגָ֗יו refers back to נָהָ֗ר (and that is a genitive phrase, of course).