Tafsir of Al-`Ankabut 29:66

The apparent meaning is that the lam in both instances is the lam of purpose (li-kay), meaning: "They associate partners [with God] in order to be ungrateful for the blessing of salvation We have granted them, and in order to enjoy their gathering upon the worship of idols and their mutual affection for it." Thus, polytheism is the cause of this ingratitude, and the lam of purpose was prefixed to its consequence to treat it as if it were their goal; it is, in reality, the lam of consequence (lam al-aqibah).

It is also said: The lam in both cases is the lam of command (lam al-amr). The command to be ungrateful and to enjoy is a metaphor for abandonment, forsaking, and threatening, similar to when you say in anger to someone who defies you: "Do as you wish."

This is supported by the reading of Ibn Kathir, al-A’mash, Hamzah, and al-Kisa’i—{وَلْيَتَمَتَّعُوا} (wa-l-yatamatta’u)—with the sukun on the lam. For the lam of purpose (lam al-kay) does not take a sukun. If the second is, for this reason, the lam of command, then the first is like it, so that the conjunction is clear. To differentiate between them necessitates the laborious interpretation suggested by Abu Hayyan, that it is a conjunction of one statement upon another, not a conjunction of one verb upon another.

His saying—the Exalted—{فَسَوْفَ يَعْلَمُونَ} (Then they shall know), meaning the consequence of that when they are punished for it on the Day of Resurrection, reinforces the interpretation of a threat.