Tafsir of Az-Zukhruf 43:47

(i.e., laughter took them by surprise upon seeing them; meaning they mocked them the moment they saw them, without reflecting upon them.)

In al-Kashshaf, it is permissible for the response to "When" (lamma) to be the "Behold" of surprise (idha al-mufajahah), because the verb of surprise is implied with it, and it is the operative agent of the accusative case in its place. It is as if it were said: "But when he brought them Our signs, they were surprised at the time of their laughing." Thus, the response according to him is that verb, which is the agent operating on "When" (lamma). He estimated it in the past tense because that is what is known in its response, and idha (behold) is the direct object, not an adverb.

Abu Hayyan said: "We know of no grammarian who holds the view that this man holds—that 'behold' (idha of surprise) is in the accusative case due to an implied verb of surprise. Rather, there are three schools of thought regarding it:

1. It is a particle, and therefore does not require an operative agent.
2. It is a spatial adverb. If a predicate is explicitly stated after the noun that follows it, that predicate is the agent operating on it; for example: Kharajtu fa-idha Zaydun qa'imun (I went out and behold, Zayd is standing). Thus, qa'imun (standing) is the one that puts it in the accusative, and the meaning is: 'I went out and found the place where I went out, Zayd is standing.'
3. It is a temporal adverb, and the agent operating on it is also the predicate; as if it were said: 'In the time in which I went out, Zayd is standing.' If no predicate is mentioned after the noun, or if a noun is mentioned in the accusative as a state (hal), then idha acts as the predicate of the subject. If the subject is a concrete entity (jutha) and we say idha is a spatial adverb, the matter is clear. If we say it is a temporal adverb, the expression implies an omitted possessive construction (mudaf), meaning: 'In the time of Zayd's presence.' Furthermore, the 'surprise' he claims is not indicated by the meaning to be of the speaker's own doing, but rather it indicates that it occurs within the speech that contains it. You say: 'I went out and behold, the lion,' so the meaning is 'the lion surprised me,' not 'I surprised the lion.'" End quote.

Al-Khafaji said: "What was said regarding its being in the accusative due to an implied verb of surprise, in this manner, is something no grammarian has said; it should not be given attention." Its details are in the commentaries of al-Mughni.