Tafsir of Ar-Rum 30:51

{Then they saw it}: They saw the effect of God’s mercy. For God’s mercy is the rain, and its effect is the vegetation. As for those who read it in the plural [i.e., āthār], the pronoun refers back to its meaning, because the meaning of "the effects of mercy" is vegetation. The noun "vegetation" (nabāt) applies to both a little and a lot, as it is an infinitive used to name that which grows.

{And if We sent a wind}: The lām is the one that prepares for an oath (al-muwaṭṭi’ah), entering upon the conditional particle. {They would remain} is the response to the oath, which serves in place of both responses—that is, the response to the oath and the response to the condition. Its meaning is: "They would certainly remain."

God Almighty censures them because, when He withholds rain from them, they despair of His mercy and hang their heads in silence. Yet, when He touches them with His mercy and provides them with rain, they rejoice and exult. Then, if He sends a wind that strikes their crops with yellowing, they clamor and deny God’s favor. In all these states, they are in a blameworthy condition: they should have relied upon God and His bounty, but they despaired; they should have thanked Him for His favor and praised Him, but they did nothing more than rejoice and exult; and they should have been patient during His trial, but they denied Him.

The wind that causes the vegetation to turn yellow: it may be a hot wind (ḥarūr) or a cold, biting wind (ḥarjaf). Both are things that cause vegetation to wither and become dry stubble. He said "yellowing" (muṣfarran) because that is a yellowing that occurs newly. It is also said: they saw the clouds yellow, for when they are such, they do not rain.

{God is the One who created you from weakness, then made after weakness strength, then made after strength weakness and gray hair. He creates what He wills, and He is the Knowing, the Powerful.}