Learn Icelandic: Þetta er gamalt hús.

In Icelandic, adjectives change their endings to match the gender of the noun they describe. The noun hús is neuter. The dictionary form of the adjective for old is gamall (which is the masculine form), but here we must use the neuter form gamalt so it agrees with hús.

Why does the spelling change from double L in gamall to just one L in gamalt?

To make a masculine adjective neuter, you usually add a -t to the end. However, Icelandic pronunciation and spelling rules do not like having a double L right before a T. Because of this, the double -ll drops one L to become -lt, resulting in gamalt.

Why do we use þetta for this instead of þessi?

Just like adjectives, pointing words like this must agree with the gender of the noun. Because hús is a neuter noun, we use the neuter form þetta. If the noun were masculine or feminine, we would use þessi.

Where is the word for an in this sentence?

Icelandic does not have an indefinite article (the words a or an in English). Therefore, gamalt hús translates directly to both old house and an old house depending on what sounds natural in English.