Learn Icelandic: Í gær var veðrið kalt. - Yesterday the weather was cold.

Icelandic uses the "V2 rule", meaning the main verb is almost always the second piece of information in a sentence. Since the sentence starts with the time phrase Í gær (yesterday), the verb var (was) has to come immediately next, pushing the subject veðrið (the weather) into the third spot.

Why do we use var instead of er in this sentence?

var is the simple past tense of vera (to be) for "he", "she", and "it". Because we are talking about veðrið (the weather - an "it") in the past (Í gær), we use var (was) instead of the present tense er (is).

I learned that "cold" is kaldur. Why is it spelled kalt here?

Adjectives in Icelandic change their endings to match the gender of the noun they describe. The word for weather, veður, is a neuter noun. To make the masculine dictionary form kaldur match a neuter noun, it drops the "-ur" and changes to kalt.

Where is the word for "the" in veðrið?

In Icelandic, "the" isn't a separate word that goes before the noun like it is in English. Instead, it gets attached to the end. The base word for "weather" is veður. By adding the definite neuter ending -ið, it becomes veðrið (the weather).