Several prepositions can cause either þolfall or þágufall depending on context.
A more general rule is that:
  • Þolfall indicates that the subject is moving relative to the object.
  • Þágufall indicates that the subject is not moving relative to the object.

Ég fór í _____ (úlpan) mína og fór svo út í búð í ______ (úlpan) minni.
Explanation:
  • Ég fór í úlpuna (þolfall) means “I put my jacket on” because þolfall implies you are going into something. The subject is moving relative to the object, the object is static.
  • Ég fór í úlpunni (þágufall) means “I went somewhere wearing my jacket” because þágufall implies you are already inside something. The subject is not moving relative to the object.

Ég fór í bílinn minn og fór svo í vinnuna í bílnum mínum.
Ég fór í _____ (bíllinn) minn og fór svo í _____ (vinnan) í _____ (bíllinn) mínum.
Explanation:
  • When you go into your car, you (the subject) are moving into the car (the object). You are moving relative to the car (the object).
  • When go you somewhere driving your car, you (the subject) are already inside the car (the object). You are not moving relative to the car (the object).
  • When you go to work, you (the subject) are moving into the job location (the object). You are moving relative to the job location (the object). You are not already inside the job location.

Ég hljóp í skólann og hljóp svo í skólanum.
Ég hljóp í ______ og hljóp svo í _______. (I ran to school. Then I ran at school)
Explanation:
  • Í skólann (þolfall) implies you are moving into the object.
  • Í skólanum (þágufall) implies you are already inside the object.