Blocking and push up
Holmberg’s (1999) formulation of Holmberg’s Generalization states that Scandinavian object shift cannot cross any phonologically realized VP-internal material. This correctly predicts that object shift may not apply in, e.g., embedded clauses in Danish: since in these languages V-to-I applies in main clauses only, the main verb occupies a VP-internal position in embedded clauses, and object shift would therefore violate HG. Generally, this is considered the end of the story, but it is not as HG can in principle be satisfied in two ways: either the verb blocks object shift, or object shift pushes the verb up into the I-position. A full explanation therefore requires an answer to the question of why the latter option is not chosen in Danish.