He is wiser than I thought. Here, the underlined part is a/an-

Whence(কোথা থেকে), wherever(যেখানে), than, while, whenever,(যখনই), because, as, since, that, so that, in order that, lest, such, if, unless, incase, whether, on condition that, provided, provided that, if, unless, though, although, even if, till, until, before, after ইত্যাদি দ্বারা Underlined clause টি শুরু হলে তা Adverbial clause হয়.