[spyrja]

I agree with Dr. Walkden here. While it was used as an interjection at times (just as it is today when someone exclaims, "What?!") in the context of the opening line of Beowulf "hwæt" is more likely being used to reformulate a statement in order to convey a sense of emphasis. An example in modern English would be something like, rather than saying "That was a gorgeous sunset!", one says "What a gorgeous sunset that was!". (Notice that the verb has now moved to the end of the sentence. In fact if you look at the last word of the line in question, we have the verb "fremedon" which means "performed", so indeed the placement of "what" at the beginning of the line facilitates the restructuring of the sentence in such a way that makes it "sounds right".)

[bccdee]

Yeah—I'm reminded of the opening sentence of Slaughterhouse Five, which goes, "Listen: Billy Pilgrim has come unstuck in time." Kurt Vonnegut isn't worried that you're not listening; he's trying to create a sense of intimacy and emphasis, as one who is taking you aside to tell you something crucially important which he fears you may dismiss out of hand.

It seems intuitive to me that the "hwaet" functioned more as a literary device than as a simple call for attention. It is, after all, a poem.

[mc32]

People are saying that the interjection interpretation is influenced by its use as interjection in Shakespeare’s time. By that time what/hwæt was being used differently than the way it was when Beowulf was authored hundreds of years before.

[spyrja]

Nevertheless I do think it is safe to say that such interjections were used, at least on a day-to-day basis. (Bear in mind that relatively few Old English texts survive to this day and almost certainly not many were produced to begin with. Old Norse itself was for the most part a spoken language, unlike Latin for example, and its predecessor which developed in England only started to be written down because of external influences.) Point is, all of the Nordic languages employ interjections akin to "Ah!", "Oh!", "Why?!", "Indeed!", "How?!", etc. So there really isn't any reason to think that such things wouldn't exist in OE as well.

[vintermann]

Those aren't interjections in other Germanic languages, they're called modal particles. Norwegian:

"Det er sant." That is true. "Det er vel sant." That's true, I suppose (resigned). "Det er nok sant." That's true, I suppose (serious). "Det er da sant." That's true, come on.

German also has them, though I'm not confident enough to explain the fine difference between "Das ist wahr" and "Das ist noch war".

English maybe has some remnants of them, but they're rare and probably a bit more archaic. You can say "That is yet true", and the "yet" there doesn't necessarily imply that you think it might not be true in the future. You probably understand what I mean if I say "That is but true" but it sounds very archaic. Usually you have to invoke a full adverb to signify mode in English ("That's actually true")

[kjellsbells]

Black American English has "true dat", which before it was a 1990s catchphrase was also used to express both emphasis as well as resignation to a state of affairs, eg "I need to get a job!" "True dat"

[WorldMaker]

It could also have been both, as well. In the immediate moment it functions as an interjection and as the rest of the sentence develops it fills in a pronoun gap. Like starting a shanty with a loud, slow single syllable to get attention (and maybe catch others on to the key) and that still being also a part of the first line of the song.