Suggested answer for Exercise 4
The verb sink is an ergative verb. Ergative verbs are similar to unaccusatives insofar as they take a theme argument, which occupies the specifier of the VP. The difference is that in sentences which contain an ergative verb, an agent can also be present. We could see this in sentence (d), where we had the agent DP John. If the agent is not there, the vP will not be projected either. It is actually the abstract light verb in v which requires an agent DP in (4). If there is no v, no agent argument is required. The only argument of the verb cannot receive Case in its base position as it did in (4) since in this structure there is no v head which can assign accusative Case. The I head is not a possible assigner of accusative Case. So the DP Jimís boat has to move to the specifier of IP, where the I head assigns nominative Case to it. The themtic verb also moves, first to the tense v, then to I to agree with the subject.