Tafsir of Al-Anbiya' 21:33

{ كل } The tanwin (nunation) here is a substitute for the genitive construction (mudaf ilayh), meaning: "all of them."

{ in an orbit they swim } The pronoun refers to the sun and the moon. What is intended by them is the genus of those that rise every day and night. They [the Arabs] made them plural due to the multiplicity of their rising points, which is the reason for using the plural forms "suns" and "moons." Otherwise, the sun is one and the moon is one. The pronoun used is the waw for rational beings because they are described by an action, which is "swimming."

If you ask: What is the grammatical position of the sentence? I say: Its position is in the accusative state as a circumstantial qualifier (hal) for the sun and the moon.

If you ask: How is it restricted to them, to the exclusion of the night and the day, in assigning the hal to them? I say: It is like your saying, "I saw Zayd and Hind adorned," and the like; when you bring an attribute that pertains to some of what the agent is connected to. From this is His saying—Exalted is He—in this same Surah: {And We gave him Isaac and Jacob as a gift} (Al-Anbiya: 72). Or, it has no grammatical position because it is an initiation (isti'naf).

If you ask: Each of the two [the sun and the moon] has its own separate orbit, so how is it said: "All of them swim in an orbit" (singular)? I say: This is like their saying, "The prince clothed them in a garment and girded them with a sword," meaning each one of them, or he clothed them and girded them with these two types. He sufficed with what indicates the genus for the sake of brevity, and because the purpose is to indicate the genus.

{ And We did not grant to any human before you immortality; so if you die, are they the immortal ones? Every soul will taste death, and We test you with evil and good as a trial, and to Us you will be returned. }