This paper aims at construing the ways two spakers of a casual conversation who are culturally different realize their commitments towards the truth of their exchanges in the conversation. The corpus of data of this study was a stretch of approximately fifty-five-minute taped-English-conversation between a native speaker (an American female) and a non-native speaker (an Indonesian male). The data were analyzed by referring to Basic System of Modality as suggested by Matthiessen (1995) in systemic functional linguistics tradition. The study reveals that NNS was more unsure towards the truth of the propositions he exchanged in the conversation compared to NS. In modalizing his propositions, NNS preferred using subjective orientation compared to NS who preferred judging something on behalf of others.Keywords: modality, modalization, modulation, systemic functional linguistics
                        
                        
                        
                        
                            
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