Learn Icelandic: Er búðin opin?

To ask a yes/no question in Icelandic, you put the verb at the very beginning of the sentence. This swaps the order of the subject and the verb. So the statement Búðin er opin (The shop is open) turns into the question Er búðin opin? (Is the shop open?).

Why is the word for shop written as búðin here?

In Icelandic, the word for "the" is usually attached to the end of the noun. The base word for "shop" is búð. When you want to say "the shop", you add the feminine ending -in to make búðin.

The dictionary says "open" is opinn. Why is it opin in this sentence?

Adjectives change their endings to match the gender of the noun they describe. Because búð is a feminine noun, the adjective changes from the default masculine form opinn to the feminine form opin.

If I want to answer "Yes, it is open", do I use the word það for "it"?

No. Because búð (shop) is a feminine noun, you must use the feminine pronoun hún (she) when referring back to it, even though it is a place and not a person. You would answer: Já, hún er opin.