كَلَّا سَوْفَ تَعْلَمُونَ
The breaking point of the sūra. The two-verse picture is finished; now the picture is intervened in.
The first (the majority) — rejection and restraint (radd and zajr). Kallā both rejects the preceding state ("no, it is not so") and restrains the one addressed from it ("give it up, stop"). It does two jobs at once: it erases a judgement and it halts a behaviour. No single English word carries it; the pair "No! Enough." is an approximate rendering.
The second — affirmation, or opening (taṣdīq / istiftāḥ). Some lexicographers say that in certain places the word may be an opening particle meaning "truly, indeed". This view is a minority one and it is not convincing for the usage in al-Takāthur, because here there plainly is something to be rejected — the state of distraction in the first verse.
An observation about the word's distribution. Kallā occurs in the Qurʾān overwhelmingly in the short, hard Meccan sūras, and almost every time it cuts off a claim or a state. Its usage in sūras such as al-ʿAlaq, al-Muddaththir, al-Maʿārij and al-Muṭaffifīn is always of this kind. The nature of the word is built upon cutting, not upon argument.
But what exactly is being rejected here? In the first two verses there was an accusation, but no claim had been openly voiced. What is kallā cutting off?
What it cuts is the assumption that was never spoken: the assumption that abundance is a gain. Those competing supposed that they were winning. Kallā erases that supposition — although it was never put into words. The particle answers a claim the one addressed has not stated but is living.
This is how kallā typically works in the Qurʾān: it takes aim at claims that have been built not as speech but as behaviour.
In Arabic there are two instruments for conveying the future: the سـ (sīn) attached to the head of the verb, and the separately written سَوْفَ (sawfa).
According to the majority of the lexicographers the difference between them is this: sīn conveys the near future, sawfa a more distant and wider one (tarākhī — the placing of a distance in between).
In the context of a threat this distance is not a lightening but an aggravation. Between a threat that comes "right now" and one that comes "one day, without fail", the second is generally the more wearing: its coming is certain, its time is not. Uncertainty lengthens the life of a threat.
Sawfa also shows that a respite is being given. The punishment is not being carried out at this moment — there is a stretch of time in between. What that time is for is not said; but that the sūra repeats the same sentence in the fourth verse suggests this gap has not been left idly.
This is the same technique as in the first verse. There, "abundance of what" was not said; here, "knowledge of what" is not said. At two critical points the sūra leaves the one addressed alone with a gap.
In rhetoric this is one of the known ways of enlarging a threat: the punishment not named is greater than the one named, because it has no limit. When it is said "you will see such and such", the mind of the one addressed draws a boundary. When it is said "you will see", it cannot.
That the verb has been chosen from the root of knowledge is meaningful too. The sūra does not say "you will be punished" or "you will be sorry". It says "you will know". The name of the problem is thus set down from the outset: this is a problem of knowledge. Man is not merely doing something wrong; he knows something wrong — or rather, he does not know the right of it. The fifth verse of the sūra will carry this diagnosis on openly: "If only you knew with the knowledge of certainty."
This is what separates the sūra from a moral sermon. Al-Takāthur does not diagnose idleness or wickedness; it diagnoses a deficiency of knowledge.