Tafsir of Ash-Shu`ara' 26:98

(When we equated you with the Lord of the worlds)

[The word] "idh" (when) is an adverbial modifier for their being in "manifest error." It is also said: it is for an omitted [verb] indicated by the discourse, meaning: "we went astray." It is also said: it refers to the aforementioned "error," although there is a technical weakness in this, in that the described verbal noun (masdar) does not govern [the object] after the description; however, the fact that the governed element is an adverb mitigates the matter. It is also said: it is an adverbial modifier for "manifest." It is also permitted that "idh" be causal, as was said regarding the saying of the Exalted: "And it will not benefit you today—since you have done wrong—that you are partners in the punishment."

The use of the present tense form is to bring the past scene into presence; meaning: "By Allah, we were in a state of extreme, flagrant error at the time we equated you [with Him]," or "because we equated you, O idols, in terms of worthiness of worship, with the Lord of the worlds, while you are the lowest of His creatures, the most abased of them, and the most incapable."