Learn Icelandic: Kóngulóin er hættuleg. Hún situr undir vaskinum. - The spider is dangerous. It sits under the sink.

Because the spider is staying in one place. The preposition undir (under) takes the dative case when describing a static location (where something is). If the spider were moving to get under the sink (direction), undir would take the accusative case instead.

Why does the second sentence start with hún (she) if the English translation says "it"?

In Icelandic, pronouns must match the grammatical gender of the noun they replace. Because kónguló (spider) is a feminine noun, we must use the feminine pronoun hún to refer back to it, even for bugs or objects.

Why is the word for dangerous written as hættuleg instead of hættulegur?

Adjectives change their endings to match the gender of the noun they describe. Because kóngulóin (the spider) is feminine, the adjective takes the feminine form hættuleg. If it were describing a masculine noun, it would be hættulegur.

How do we get the form vaskinum for "the sink"?

The base word is vaskur, a masculine noun. In the dative case, its ending drops to become vaski. Then we add the masculine dative "the" suffix, which is -num, combining to make vaskinum.