Learn Icelandic: Svo borðum við ísinn. - Then we eat the ice cream.

This is because of the V2 (verb-second) rule! In Icelandic, the verb must be the second piece of information in a main clause. Since the sentence starts with the adverb svo (then), the verb borðum must come immediately after it, pushing the subject við to the third spot.

I thought svo meant "so". Why does it mean "then" here?

Svo is a very common word with a few meanings. When used to describe an adjective, it means "so" (like "so big"). But when used to connect events in a sequence like it is here, it means "then" or "after that".

How does the word ísinn mean "the ice cream"?

In Icelandic, the word for "the" isn't a separate word. Instead, it gets attached to the end of the noun. The base word for "ice cream" is ís. Adding the definite ending -inn makes it ísinn, which means "the ice cream".

Does the verb borða (to eat) change the form of ísinn here?

Yes! The verb borða requires the thing being eaten (the direct object) to be in the accusative case. For a masculine noun like ís, the accusative definite form is exactly the same as the dictionary form: ísinn. So it doesn't look different, but it is grammatically in the accusative case here.