Subjects in Scandinavian

One of the most fundamental changes from Old Norse to the modern Mainland Scandinavian languages is the introduction of the obligatory subject requirement. While Old Norse allowed subjectless sentences of various kinds, present-day Mainland Scandinavian requires an overt subject in finite sentences. During this transition, a grammatical subject was recruited either by a non-subject taking over the subject role, or by the new expletive filling the subject role. This change is explained as a result of the loss of the invisible pronoun pro, which in Old Norse could receive nominative case and function as a subject. The new situation with an obligatory overt subject is due to the loss of pro from the lexicon, a development which may be explained as the result of less robust input data for new generations of learners.