This thesis investigates the semantics of modal verbs in Vietnamese, in contrast with Mandarin Chinese, in terms of three categories of modal meanings, viz. deontics, epistemics and dynamics.
Regarding deontic meanings, I’ll investigate two sub-classes: obligation and permission. Deontics indicates an agent’s authority, that is, speaker’s permission of or insistence on events and also the responsibility of speakers or subjects. In epistemics, two meanings are discussed: probability and uncertainty. Both of them express the possibility or necessity of the occurrence of events. Besides, dynamics is concerned with volition and abilities of speakers or subjects.
There is no doubt that in quite different languages, most of modal verbs have more than one category, and it would appear from the glosses that in different contexts both interpretations are available for a single form. In order to probe contextual meanings of each modal verb, and the relationship among forms and meanings (one-to-many), the study goes with authentic linguistic data of the two languages. Both the symmetrical phenomena between modal positive forms and their negative counterparts, and the hierarchical relationship between modal verbs and negatives are also discussed.
The latter part of the thesis deals with the post-verbal được in Vietnamese and post-verbal de2 (得) in Mandarin Chinese. Besides some similarities in modal meanings, these two modals exist, however, quite many differences in constructions as well as the correspondence between constructions and their meanings. Via different constructions, modal meanings are rendered.
This thesis also investigates the important relation between negation and modality. Both deontics and epistemics can express either possibility or necessity. Negatives are markers with unique functions grammatically and semantically. In some cases, the positive-negative switch of modal verbs can also switch the reading between possibility and necessity. In addition with contexts and speakers’ attitudes and emotions, interpretations of negation are more complicated, and mostly show extra pragmatic meanings.

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