Short answer: Very often you don't see the patronym in a source until a person leaves their parents' home. You look at the father's first name at birth, let's say it is Eric. You standardize it to Erik and you get Eriksson as patronym/last name/LNAB for the child.
EDIT (after Axel's first comment below), long answer:
I meant the short answer above as what to do when you only have the birth record, when you have the HHE, it is more complicated:
Quote:
"Which source, then, is most reliable in determining the name of a person who lived a hundred or two hundred years ago? To be able to answer the question, we have to consider how the various sources were created and used. In the birth, marriage and death records, the priest made notes when the parishioners had their children baptized, married or buried their dead. There were solemn occasions with ceremonies and rituals, it was easy for the priest to be infected by the seriousness of the moment. People who in almost all other contexts are named Per, Olof, Kajsa or Greta are suddenly listed as Petrus, Olaus, Katarina and Margareta. This is most evident in the birth records, which thus become the least reliable source.
The household examination books worked quite differently.
Once a year the ancestors had to pass the household examination. The priest sat at a table with the book in front of him, called out the names of the congregants one by one and tested them on Christian knowledge and literacy. Then he dipped the quill into the inkhorn and noted their skills.
In other words, it was essential – for practical reasons – to note the correct forms of names in the house interrogation length, names that people recognized. The farmer Olof Johansson - always called "Södergårds-Olle" in the village - would hardly like the name Olaus, which he didn't even know he was recorded with in the birth book by the priest, knowledgeable in Latin and with a taste of the learned."
"And the best single source is probably the household examination books."
https://www.rotter.se/faktabanken/personnamn/bakgrund-namnlistan (The standard reference for swedish names, see separate answer)