Tafsir of Az-Zukhruf 43:46

"And We certainly sent Moses with Our signs to Pharaoh and his establishment, and he said, 'Indeed, I am the messenger of the Lord of the worlds.'"

The response they gave him when he said, "Indeed, I am the messenger of the Lord of the worlds," is omitted. It is indicated by His saying: "But when he brought them Our signs..." which refers to their demanding that he produce proof for his claim and manifest the sign.

"Behold, they laughed at them." Meaning: They mocked them, ridiculed them, and called them magic. "Behold" (idha) here is for sudden occurrence (al-mufaja'ah).

If you ask: How is it grammatically permissible for "when" (lamma) to be answered by "behold" (idha) of sudden occurrence?

I say: It is because the verb of sudden occurrence is implied with it, which acts as the governor of the accusative case in its position. It is as if it were said: "So when he brought them Our signs, they suddenly [began] their laughing."

"And We did not show them a sign except that it was greater than its sister, and We seized them with the torment that perhaps they might return."