"He has a very big dog."
Translation:Han har en jättestor hund.
In this sentence, "väldigt" (very) is an adverb that describes "stor" (big). In Swedish, adverbs don't change forms for different genders. This is because they describe adjectives or other adverbs, not the noun. You would never take off the t if it was being used as an adverb meaning very. "Väldig" without the t is an adjective that means enormous.
I assume you got this as a multiple choice question and the correct answer you saw was not Han har en jättestor hund. That's because the options with mycket stor or väldigt stor are just as good translations of very big, so either one of them, two of them or even all three could be shown in a multiple choice question.
My secondary school taught a couple of weeks of Scots, I remember the teacher saying that there are strong Germanic roots to a lot of the language, as well as some Gaelic. Thank you for reminding me of that - I've been trying to pin some Swedish words to similar ones in English to help me learn them, I forgot that I could do the same with Scots :)
Absolutely. All of Han har en mycket stor hund, Han har en väldigt stor hund, and Han har en jättestor hund are accepted answers and if you get them in a multiple choice here, you may even have to choose all three of them.
mycket stor is the most formal of the three versions, jättestor is the most colloquial, and väldigt stor is the most neutral way of putting it.