[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"