Tafsir of Ash-Shu`ara' 26:4

"If We willed, We could send down..." He meant: A sign that compels belief and restricts them to it.

"{So their necks would remain...}" This is conjoined to the result (jazā’) which is "We would send down," for if it were said "We sent down" (anzalnā), it would be correct. Its parallel is: "So I may give charity and be (wa-akūn) among the righteous," as if it were said: "I will give charity." It has been recited: "If We willed, We would have sent down" (law shi’nā la-anzalnā). It has also been recited: "So their necks would remain" (fa-taẓalla a‘nāquhum).

If you ask: How is it grammatically correct for "submissive" (khāḍi‘īn—masculine plural) to be the predicate for "necks" (a‘nāq—feminine plural)? I say: The original structure of the speech is "So they would remain submissive to it" (fa-ẓallū lahā khāḍi‘īn). Then "necks" was inserted to clarify the location of the submission, and the speech was left in its original state, similar to the saying: "The people of Yamama have departed" (dhahabat ahl al-Yamāma), as if "the people" were not mentioned. Or, because they were described with submission—which is an attribute of rational beings—the term "submissive" (khāḍi‘īn) was used, as in His saying: "Prostrating to me" (lī sājidīn).

It is said: "The necks of the people" refers to their leaders and chiefs. They are likened to necks, just as they are called "the heads," "the forelocks," and "the chests." A poet said: In a gathering of the forelocks of the people, witnessed.

It is also said: It means groups of people. It is said: "A neck (‘unuq) of people came to us," meaning a troop of them. It has been recited: "So their necks would remain submissive to it" (fa-taẓallat a‘nāquhum lahā khāḍi‘ah).

From Ibn Abbas (may Allah be pleased with them both): This verse was revealed regarding us and the Umayyads. He said: "We will eventually have the state over them, so their necks will submit to us after being difficult, and humiliation will overtake them after their glory."

"{And no mention comes to them from the Most Merciful, newly revealed, except that they turn away from it. For they have denied, so the tidings of what they used to mock will come to them.}"