This study describes the thematic roles of Somali; a language that manifests unorthodox grammatical structures due to the use of the focus particle. A mapping of some language features related to the semantic-syntactic interface is carried out. The effect of the meaning of the lexical items is tested to whether it affects the syntactic structures of the sentences. It was found that the word order of Somali is determined by pragmatic and sociolinguistics factors. The focus particle of Somali plays a key-role in the Surface structure of the sentences. However; the thematic roles of the language appear not to be influenced by the grammatical manifestation of the language at the surface structure. Rather, they are determined at the deep structure level. Somali, being a pro-drop language, shows that predicates require obligatory external arguments while internal arguments are determined by the verb class and kind. Some structures show that certain verbs require PPs as obligatory arguments.