Abstract
This thesis shows that the potential de, the demand dei and the permissive de are modals. They occur and behave differently. I propose that the potential de and the permissive de are clitics because of their prosodic lightness, whereas the deontic dei is not a clitic and can stand alone due to its prosodic heaviness. For syntactic position, the modal de constructions from high to low are the deontic dei, the permissive de and the potential de, respectively. The deontic dei takes a higher AspP complement; the permissive de as the head of ModP takes a VP complement; the potential de as the head of DeP takes a VP complement. Moreover, I also show the semantic properties of the modal de constructions. The potential de indicates possibility; the deontic dei denotes a deontic necessity, a demand expressed by the speaker; the permissive de usually expresses prohibition imposed by the speaker with the present of the negation bu.