A decade ago we briefly discussed the famous first word of Beowulf, “Hwaet,” and the various ways translators have dealt with it. Now it seems all previous discussions may have been rendered obsolete by a paper (pdf) by George Walkden, “The status of hwæt in Old English” (English Language and Linguistics 17.3: 465–488); Dave Wilton at Wordorigins.org has a good discussion in which he says:
Walkden makes a compelling, but by no means ironclad, case that hwæt is not an interjection or an adverb, rather it has no independent meaning from the clause it appears in. It combines with the remainder of the clause to produce an exclamatory effect. In this way it is similar to the modern English how, as in “how you’ve changed!” According to Walkden’s conclusion, the opening line of Beowulf would read:
How we have heard of the glory…
[…]The core of Walkden’s argument is a syntactic analysis of the use of hwæt and its Old Saxon cognate huat. He finds that in clauses beginning with hwæt/huat, the verb tends to appear in a later position than would normally be the case for a root clause. Instead, hwæt-clauses follow the pattern expected of a dependent clause. If hwæt were an interjection independent of the clause, it should not influence word order.
As I said in the Wordorigins thread, on a first reading it’s pretty convincing.