Learn Icelandic: Hvernig er súpan?

Yes, hvernig means "how" and is used to ask about the state or quality of something. Just like in English, you can use it to ask how food tastes, how someone is feeling, or how something works.

Why couldn't I use hvaða in this question?

The word hvaða means "which" or "what" and is used right before a noun to identify something, like in hvaða súpa (which soup). Because we want to know about the soup's quality rather than pointing out a specific bowl of soup, we must use hvernig (how).

Where is the word for "the" in this sentence?

In Icelandic, "the" is a suffix added to the end of the noun instead of being a separate word. The base noun is súpa (soup). To say "the soup", you add an -n to the end to make it súpan.

Why is the word order hvernig er instead of hvernig súpan er?

Just like in English, the verb in an Icelandic question usually swaps places with the subject. Right after the question word hvernig (how), you place the verb er (is), followed by what you are asking about, súpan (the soup).