Tafsir of Ar-Rum 30:36

(And when We let the people taste mercy), meaning a blessing such as health, abundance, and the like, (they rejoice in it), meaning out of insolence and arrogance; for this is the blameworthy rejoicing, as opposed to the rejoicing that is praise and gratitude, which is what is intended in His saying—Exalted is He: "Say, in the bounty of Allah and in His mercy—in that let them rejoice." The Imam said: The blameworthy rejoicing is rejoicing in the mercy itself, while the praiseworthy rejoicing is rejoicing in the mercy of Allah the Exalted, in that it is attributed to Allah the Exalted.

(And if an evil befalls them), meaning a hardship, (on account of what their hands have sent forth), meaning because of the ill-fortune of their sins, (behold, they despair), meaning they suddenly fall into despair of His mercy—Exalted and Majestic is He.

The use of "when" (idha) in the first instance is to denote the certainty and frequency of mercy, unlike its counterpart. In attributing mercy to Him—Exalted is He—rather than evil, there is a lesson for the servants that evil should not be attributed to Him, Glory be to Him, and this is frequent, such as in His saying—Exalted is He: "You have bestowed favor" and "those who have incurred wrath" in Al-Fatihah. The omission of the cause for the tasting of mercy and the statement of the cause for the befalling of evil is an indication that the former is grace, and the latter is justice.

The use of the present tense in idha hum yaqnatun (behold, they despair) is to observe the rhyme scheme and to indicate continuity in despair.

As for "the people," they are either a group other than the first, assuming the definite article denotes a specific reference or the generic category; or they are the first group, but the first ruling is fixed for them in a state of bewilderment, such as witnessing death, while this ruling applies to them in another state, so there is no contradiction between His saying—Exalted is He: "And when affliction touches the people, they call upon their Lord, turning toward Him" and His saying—Glorified is He: "And if an evil befalls them on account of what their hands have sent forth, behold, they despair." Therefore, there is no need to resort to forced reconciliation by claiming that the verbal supplication is merely a habit and does not negate the despair of the heart. For this reason, some of those who engaged in the shedding of the blood of Uthman—may Allah be pleased with him—were heard praying during their tawaf, saying: "O Allah, forgive me, though I do not think You will do so." Or, the meaning is that they commit the deeds of the despairing, such as the preoccupation with hoarding supplies during times of high prices. It is not hidden that the element of surprise (mufaja'ah) carries a certain indication of this, so reflect upon it.

(It has been recited: yaqnitun with a kasra on the nun.)