Learn Icelandic: Barnið sofnar strax þegar ég svæfi það. - The child falls asleep immediately when I put it to sleep.

This sentence perfectly shows the difference between an intransitive verb and a transitive causative verb. Barnið sofnar uses the intransitive verb sofna, meaning the child falls asleep on its own. Ég svæfi það uses the causative verb svæfa, meaning I actively make the child sleep (I put it to sleep).

What case does the transitive verb svæfa take?

Like many causative verbs (verbs that mean making someone do something), svæfa takes an object in the accusative case. Here, the object is það, which is the accusative form of the neuter pronoun, referring back to the child.

Why is the pronoun það used at the end instead of hann or hún for the child?

In Icelandic, pronouns must match the grammatical gender of the noun they replace. Even though a child is a person, the word barn is a neuter noun. Therefore, you must use the neuter pronoun það to refer to it, regardless of the child's actual gender.

I notice sofnar has an "o" but svæfi has an "æ". Is that a common pattern for these verb pairs?