Tafsir of As-Saff 61:12

(He will forgive you your sins) is a response to the command implied by the declarative phrasing, as in the saying: "Let a man fear Allah the Exalted and do good; he will be rewarded for it." Or it is a response to a conditional or interrogative clause implied by the discourse. The estimation is: "If you believe and strive, He will forgive you," or "Will you accept that I guide you?" or "Will you trade through faith and striving? He will forgive you."

Al-Farra’ said: It is a response to the mentioned interrogation, "Shall I guide you?" This was critiqued on the grounds that mere guidance does not necessitate forgiveness. It was answered that it is like the saying of the Exalted: "Say to My servants who have believed, [that] they should establish prayer." Scholars said regarding that: Because the command was directed to a believer of firm faith, it was a presumption of compliance, so it was treated as something confirmed to occur. Thus, it is said here: Since the guidance was a presumption of that [faith and striving], it was placed in the position of something confirmed. This is supported by "if you should know," for whoever possesses intellect, when his Master guides him to what is best for him, will not abandon it. The claim that there is a difference between that [verse]—in terms of the honorific attribution—and this one—in terms of admonition—is said to be not apparent, so contemplate this.

In truth, Al-Farra’s interpretation is not free from being far-fetched. As for what was said—that the sentence is an incipit (newly commenced) to clarify that this is better for them, and that yaghfir (He will forgive) is in the nominative case with its final vowel suppressed, just as the final vowel of ushrab was suppressed in the verse: "Then today ushrab (he drank)... not incurring sin from Allah nor being an intruder"—this is baseless, as they have explicitly stated that such [suppression] is a poetic necessity.

{And He will admit you into gardens beneath which rivers flow and pleasant dwellings}—that is, pure, sanctified, and delightful. This is an indication of their excellence in themselves. His saying, the Exalted: {in gardens of perpetual residence} is an indication of their excellence in terms of their location. {That}—meaning what was mentioned of forgiveness and what was conjoined to it—{is the great attainment}, meaning that there is no attainment beyond it.