That is a beginning of the news of the towns. We narrate it to you, news after news. That is: that news is some of the news of the destroyed towns. The pronoun refers to the towns, meaning: some of them remain and some have lost their traces, like the crops standing on their stalks and those that have been harvested. If you say: what is the position of this sentence? I say: it is an independent statement with no position. And We did not wrong them by destroying them, but they wronged themselves by committing what led to their destruction. So their gods did not avail them, nor could they repel the punishment of Allah from them. They call upon them, worship them, and this is a narration of a past state. And 'when' is an adverbial clause connected to what availed them. The command of your Lord is His punishment and His vengeance, which brings about loss. It is said: 'tabb' if he loses. And 'tabbah' is when someone else causes him to fall into loss.