How will Hashem make it a smooth rock?
Malbim: The sea will cast off its earth, and only the smooth rock underneath will remain. It will not be proper for settlement.
What is the grammatical form of "Tihyeh" and "Tibaneh"?
Radak: It is third person feminine (the verse switched from second person to third).
The verse says that it will not be rebuilt. Tzor was rebuilt!
Radak: The city Tzor (Tyre) nowadays is a different city on the coast. The old Tzur was in the sea, and now it is a smooth rock.
Malbim citing Mahari: It was rebuilt, and Alexander Mukdan destroyed it; this Nevu'ah teaches that it will not be rebuilt after that 1 .
Malbim: This is wrong. The Navi discusses the Churban via Nebuchadnetzar! It was rebuilt elsewhere (like Radak said ? refer to 26:14:3:1). Ir ha'Nidachas may not be rebuilt (Devarim 13:17), i.e. like it used to be, but one may build it to be gardens and orchards (Sanhedrin 111b). (All the more so, one may build a city elsewhere with the name of the destroyed city! ? PF)