Learn Icelandic: Hér er kakan.

Because the verb vera (to be) never takes a direct object. It acts like an equals sign linking things together. Any noun that follows er (is) stays in the nominative (subject) case, which is why we use kakan.

How does kakan mean 'the cake' instead of just 'cake'?

In Icelandic, the word for 'the' isn't a separate word. Instead, it gets attached to the end of the noun. The basic bare word for 'cake' is kaka. Adding -n to the end makes it kakan (the cake).

Why is the word order Hér er kakan instead of Hér kakan er?

Icelandic uses a strict rule where the verb is almost always the second piece of information in a sentence. Since the adverb Hér (Here) takes up the first spot, the verb er (is) must come immediately next, pushing the subject kakan to the end.