Meaning of "d-n" in "'T is strange—the Hebrew noun which means 'I am,' the English always used to govern d—n"

Question

From Byron's Don Juan:

She liked the English and the Hebrew tongue,
And said there was analogy between 'em;
She proved it somehow out of sacred song,
But I must leave the proofs to those who've seen 'em;
But this I heard her say, and can't be wrong
And all may think which way their judgments lean 'em,
"'T is strange—the Hebrew noun which means 'I am,'
The English always used to govern d—n."

What is this word? Darn? But what could govern darn possibly mean?

And "darn" would not rhyme. Anyway, I'm not a native speaker, so it's doubly hard for me.

Answer 1

This is a very subtle piece of wordplay, so it makes for an excellent question. The meaning, believe it or not, is God, and your answer "damn". This is analysed in An Ingenious Jest in Byron's "Don Juan", a paper by John I. Ades in Papers on Language and Literature 24(4) (1988), p. 446.

The Hebrew sacred name for God is YHVH, usually vocalized as "Yahweh" or "Yahveh." In Byron's day (as in many scholarly circles yet) it was thought that "Yahweh" derives from a form of the Hebrew verb "to be" used in Exodus 3:14-15, where, in reply to Moses's request for God's name, God replies, "I am". Thus, mutatis mutandis, the "I am" that Byron wittily asserts the English use to "govern d--n" is, of course, "God"; producing the mild oath, "goddamn." We see here the extraordinary lengths to which Byron would go to set up a jest. In stanza XIV Byron neatly accomplishes three objectives: he displays his etymological wit, he trivializes Donna Inez's (Lady Byron's) learning, and he fixes a satirical barb in the hide of the English gentry for habitually taking the Lord's name in vain.

In short: "the Hebrew noun which means 'I am'" is God, which is also used by the English to "govern damn" in the phrase god-damn. (Presumably it's damn, not darn as you guessed, to rhyme with "I am" in the previous line.)

The interpretation of YHVH as "I am" is supported by the English Standard Version of the Bible itself, as reproduced here:

The word Lord, when spelled with capital letters, stands for the divine name, YHWH, which is here connected with the verb hayah, “to be” in verse 14.

Answer 2

This is too long for a comment, but I'd like to add to @Rand al'Thor's excellent answer a bit more about the use of "govern" in talking about grammar. Quoting from the online OED's (paywalled) definition of "govern":

13. transitive. Grammar. Of a word, esp. a verb or a preposition: to have (a word or a case) depending on it; to require (a certain case or mood) in a dependent word. Also: (of a subject) to determine the number and person of (the verb). Also occasionally intransitive.

In languages with cases, a verb or preposition may be said to "govern" a certain case, such as the dative or the accusative. It may be less usual to speak, as Byron does here, of a subject "governing" a verb, but the OED cites these examples:

1835   C. Follen Pract. Gram. German Lang. ii. 186    Here, der Va'ter is the subject, which
           governs the verb, that is to say, the verb must agree with it, in number and person.

1924   S. Perry Making Lett. talk Business 153    If we are to use verbs correctly, we must
           remember that the subject always governs the verb.