Allah (swt) says:
I swear by the Day of Resurrection
The word “laa” is usually used as a nafi (negation) and means “No.” But if that were the case in this ayah then the meaning would be the opposite of what it is actually intended because here Allah (swt) is swearing an oath by the Day of Resurrection. So why is the word laa used in this ayah?
The explanations given for this are as follows:
1) Ziyaadah fil kalaam
Meaning that it is simply an additional word used (without having its meaning intended) to increase the beauty of the recitation.
2) Radd ‘alaihim
Another interpretation is that it is said in response to those who deny the Resurrection. It is as if Allah (swt) is saying to those who deny the Resurrection “Laa, the matter is not as you have determined.” And this kind of speech is acceptable in Arabic where you might find someone saying:
“No by Allah I did not…” And this is in response to something which was said previously.
A third explanation is that the “laa” is used as a silah – a connection / conjunction. And it is permissible for this used to be used at the start of the Surah as all of the Quran is connected. So you might find something mentioned in one ayah and the explanation of it is found some place else.
4) Laam and not laa
Some people read this as لأُقْسِم without any alif after the laam. In this case there is no problem as here the laam is used as a ta’keed – it gives emphasis to the oath.
And there are many other ayat in the Quran similar to this one.