Learn Danish: Er I sure?

Because it describes I, which is the plural form of "you". In Danish, when an adjective describes a plural noun or pronoun, you usually add an -e to the end of it. The singular form is sur, but the plural form is sure.

Does I always mean plural "you"?

Yes! I is specifically used when talking to more than one person, like saying "you guys" or "y'all" in English. If you were only asking one person, you would use the singular pronoun du and the singular adjective: Er du sur?

Doesn't the word sure normally mean "sour"?

The base word sur does mean "sour" when talking about food, like a lemon. However, when you use it to describe a person, it means they are "mad", "angry", or "grumpy".

Why does the sentence start with the verb Er?

Just like in English, we swap the verb (er) and the subject (I) to turn a statement into a yes/no question. The statement "You are mad" is I er sure, so the question "Are you mad?" becomes Er I sure?