Tafsir of Al-Hijr 15:71

"He said: These are my daughters..." (meaning the women of the nation, or his daughters in reality, and the discussion regarding that has preceded). The demonstrative pronoun is the subject (mubtada’) and "my daughters" (banati) is its predicate (khabar). There is an ellipsis in the statement, meaning: "so marry them." It is also permitted that "my daughters" is an appositive (badal) or an explanatory clause (bayan), with the predicate being elliptical, meaning: "they are purer for you," as in the other verse. It is also possible that "these" is in the accusative case due to an implied verb, meaning: "marry my daughters." The most apparent is the first interpretation.

"...if you are to do [it]."

(He expressed doubt regarding their acceptance of his words, as if he were saying: If you do what I tell you—though I do not think you will do it). It is also said: "If you intend to fulfill your desires through what Allah the Exalted has made lawful, rather than what He has forbidden." The first interpretation is the most sound, as stated in al-Kashf. In the Shihabiyyah marginal notes, it is stated that it is more fitting regarding the expression of doubt, while the work of some scholars implies the preference for the second [interpretation] because of its immediacy to the action. According to both interpretations, the object [of the verb "doing"] is implied. It is also permitted to consider the descriptive quality as being in the position of an intransitive verb, and the response to the conditional clause is elliptical, meaning: "then that is better for you," or "then fulfill that [desire through them]."