Breakdown of Þau fara í afmælið með lækninum.
læknirinn
the doctor
þau
they (n)
fara
to go
í
in / into / to (before the hour, in telling time) / per (in frequency expressions)
Questions & Answers about Þau fara í afmælið með lækninum.
Why is the word for "the doctor" spelled lækninum here instead of læknirinn?
This is because of the preposition með. When með means "with" (as in going together with someone), the word following it must be in the dative case. The standard dictionary form for "the doctor" is læknirinn, but in the dative case, it changes to lækninum.
Why is it í afmælið (accusative) but með lækninum (dative)?
Different prepositions trigger different cases! The preposition með takes the dative case here to show accompaniment. However, the preposition í takes the accusative case because the sentence describes movement towards a destination (going to the party).
Does fara í afmælið literally mean they are going "into" the birthday?
Yes, literally it means "go into the birthday." In English, we say we go "to" a birthday party, but Icelandic uses í (in/into) for attending events and gatherings. Notice that the word for "party" is just understood from the word afmæli (birthday).