Learn Icelandic: Er þessi leið mjög vond?

This is a simple yes/no question, so it doesn't need a question word like hvaða (which/what) or hvernig (how). Just like in English, you form a yes/no question by moving the verb to the front. The statement would be Þessi leið er mjög vond (This route is very bad), and putting the verb first makes it the question Er þessi leið mjög vond?.

Why does the sentence use "þessi" instead of the default "þetta" for "this"?

When "this" is attached directly to a noun, it has to match that noun's gender. The word leið (route/way) is a feminine noun, so we must use the feminine form þessi. You generally only use þetta if the noun is neuter, or if you are pointing to something without naming the specific object (like saying "This is a bad route" instead of "This route is bad").

Does the adjective "vond" also have to match the noun's gender?

Exactly! Because leið is feminine, the adjective for "bad" takes its feminine form, vond. If the noun were masculine, it would be vondur, and if it were neuter, it would be vont.

Is "vond" the normal word to describe a physical path or road?

Yes! While vondur translates to "bad" or even "evil" when talking about people, when it is used for physical things like roads (leið) or the weather, it means "rough," "nasty," or in poor condition.