Tafsir of Ya seen 36:45

(And when it is said to them...) This is an explanation of their turning away from the revealed verses, following the explanation of their turning away from the cosmic signs which they were witnessing and regarding which they did not reflect. That is, when it is said to the people of Mecca—by way of warning through what has been revealed of signs or otherwise—{ Fear what is before you }—Qatadah and Muqatil said: "That is, the punishment of the nations that were before you," and the intent is "Fear the like of their punishment," { and what is behind you }—"That is, the punishment of the Hereafter." Mujahid said in one narration the opposite of that, and it came from him in another narration that "what is before them" is what has preceded of their sins, and "what is behind them" is what will come of them. Al-Hasan held a similar view. It was also said: "What is before them" are the calamities of the sky, and "what is behind them" are the catastrophes of the earth. It was also said: "What is before them" are the adversities from where they anticipate, and "what is behind them" are the adversities from where they do not anticipate. The crux of the matter, according to what has been said, is: "Fear the punishment, or fear that which results in the punishment."

{ That you may receive mercy }

This is a circumstantial qualifier (hal) for the [plural] pronoun in "Fear," or it is the ultimate objective thereof, meaning: "Hoping that you may be shown mercy," or "So that you may be shown mercy." Mercy has been interpreted as salvation from the punishment. The response to "when" (idha) is omitted, confident in its being understood from His saying—the Exalted—