Suggested answer for Exercise 4

In the sentence John’s message arrived the verb arrive is an unaccusative verb, which takes only one argument, assigning a theme thematic role to it. The argument is in the specifier position of the VP in the D-structure. Since DPs need Case, it has to move to the specifier of the IP, where it gets nominative Case from the finite I head. The higher vP in the structure hosts the tense morpheme -ed, which the lexical verb picks up on its way to I.