Causative Verb Pairs
In English, we often use the same verb whether an action happens by itself or we cause it to happen to someone else (for example, "I wake up" versus "I wake him up"). In Icelandic, these are usually two distinct verbs.
The verb where an action simply happens is intransitive (it takes no object). The verb that causes the action is transitive (it takes a direct object, usually in the accusative case).
Waking Up
The verb vakna (to wake up) is intransitive. If you want to wake someone else up, you must use the transitive causative verb vekja. The person being woken up takes the accusative case.
Falling Asleep
Similarly, the act of falling asleep uses the intransitive verb sofna, while putting someone to sleep uses the transitive causative verb svæfa.
Calming Down and the Middle Voice
Sometimes, the intransitive version of a verb is formed by adding the -st suffix to create the middle voice. For example, róa means to calm someone down (transitive), while róast means to calm down yourself (intransitive). If a child is fussy (óvær), you might also need to comfort (hugga) them.
To soothe a fussy kid, a babysitter (barnapían) has a few tools: a pacifier (snuddan), a teddy bear (bangsinn), or a baby bottle (pelinn). When these are used as the direct object of a transitive verb, they take the accusative case.