"Hvad skete der?"

Translation: What happened?

[rhhpk]

Why "skete der" and not just "skete"?
In the previous sentence, the "der" seems not only unnecessary, but strange, since there is a place mentioned.

[Mathilde.Buch]

I can't give you an exact answer to that (i'm a native speaker), but that's just how the language works

[ilmolleggi]

I am no native speaker but my guess is it works like "there is" where the adverb becomes part of the verb and no longer means anything on its own (think about "There's a cat here" -> the cat is not there it's here, so the there only serves the purpose of strengthening the short is)

[geometry667721]

That's helpful, thank you!

[Ektoraskan]

Please, what is the infinitive form of this verb?

[Xneb]

A good place to find this is on Den Danske Ordbog. For example, if you search "skete" it comes up with the infinitive of the verb "ske" with all its forms below in the "Bøjning" section (present, simple past and past participle)
(N.B. of course you have to make sure it's not sent you to a different word, but you can see all the different entries for a word on the right hand side under "Opslagsord" and it's just a matter of selecting the correct one with "vb." following it)

[Ektoraskan]

Oh wow, thank you so much!

[tonkotsuLover]

What does bøjning translate to?

[boydi14]

inflection(s), I believe

[Respro]

... and the main tenses?