There is a category of verbs called impersonal verbs. These are verbs which do not conjugate like most verbs, but always look like the third person singular (he/she/it) conjugation.
Recall that the subject is often defined as the doer: the one doing whatever is happening. This is not always accurate, as is apparent in sentences like “John thinks khaki pants are cool” or “John is cold”. John is clearly the subject, yet he can hardly be said to be doing anything at all. In these sentences John is simply holding an opinion or experiencing something. This sort of subject is fittingly labeled an experiencer.
Impersonal verbs always take an experiencer as a subject. Experiencer subjects may take cases other than the expected nominative. These can be the accusative or dative, but never the genitive.
There are faint remnants of impersonal verbs and experiencer subjects in English, such as methinks in Shakespeare’s “The lady doth protest too much, methinks.” Thinking is not an action so it used to take an experiencer subject, me, and the form of the verb was thinks just like he/she/it thinks.
Unfortunately, there is no simple way to tell which impersonal verb takes which case – they need to be memorised. Here is a list of the most common impersonal verbs. Some of the translations may feel clunky in English – having an experiencer subject allows for a lot of nuance that is often hard to translate simply.
By definition no impersonal verb takes the nominative case. However, two personal verbs do take experiencer subjects, despite the tendency that impersonal verbs and experiencer subjects go together. These are included for completeness’ sake.
Að hlakka til > to look forward to
Að kvíða > to be anxious, to have anxiety
Here are eight of the most common impersonal verbs which take the accusative case.
Að kitla > to be ticklish, to be tickled
Að minna > to think one remembers correctly
Að dreyma > to dream
Að langa > to want
Að vanta > to need
Að verkja > to feel pain
Að klæja > to feel itchy
Að svima > to be dizzy
Here are ten of the most common impersonal verbs which take the dative case.
Að vera heitt/kalt > to feel hot/cold
Að leiðast > to be bored
Að vera sama > to not care
Að líða vel / illa > to feel good / bad
Að batna > to improve
Að seinka > to be delayed
Að detta í hug > to have an idea (lit. to fall into mind)
Að finnast > to think, to find, to opine, to be of the opinion that
Að sýnast > to look a certain way (note the subject and object are flipped compared to English)
Að heyrast > to sound a certain way (note the subject and object are flipped compared to English)
Impersonal Verbs Which Take the Genitive
No impersonal verb takes the genitive case.
An impersonal verb is a verb which doesn’t conjugate for person and number but is always in the third person singular (hann/hún/það). An experiencer subject is a subject which is not doing but experiencing. Impersonal verbs always take an experiencer subject. Experiencer subjects may be in cases other than the nominative (but not the genitive). There is no way to recognise which case a particular impersonal verb takes, so they must be memorised.
The article could end with that summary. The above has been an explanation of impersonal verbs as they are supposed to be used in formal or academic speech and conforms to every style guide about proper language.
However, the Icelandic language is evolving and changing every day. One of those changes is a tendency to use the dative instead of the nominative or accusative for all experiencer subjects. This is frequently termed þágufallshneigð (dative tendency) or þágufallssýki (dative sickness) and it is, as the name implies, frequently viewed as improper or uneducated language. Nonetheless, the change has already passed in a certain regard: 100% of Icelanders have this tendency, without exception.
How can a variation already employed by all Icelanders be viewed as improper? Since it has historically been viewed as “incorrect speech”, more-educated people and people who wish to be seen as more-educated tend to (subconsciously) memorise the most common forms of a subject as a sort of exception to the rule that all impersonal verbs take the dative. This results in “misuse” of simple personal pronouns (ég, þú, hann/hún/það etc.) to stand out, while longer subjects help conceal the “misuse”.
Honum is likely to provoke the ire of grammar…enthusiasts, as the accusative hann “should” be used. In the example below, however, the subject is longer and thus the "error" is likely to go unnoticed.
For the second-language learner, this can be a blessing: all impersonal verbs can be used with the dative, no memorisation necessary. However, be aware that this usage would not be appropriate in formal or academic contexts, and that people who police other people’s language will react strongly.