In this paper, we propose to study the effects of negation and modality on opinion expressions. Based on linguistic experiments informed by native speakers, we distill these effects according to the type of modality and negation. We show that each type has a specific effect on the opinion expression in its scope: both on the polarity and the strength for negation, and on the strength and/or the degree of certainty for modality. The empirical results reported in this paper provide a basis for future opinion analysis systems that have to compute the sentiment orientation at the sentence or at the clause level. The methodology we used for deriving this basis was applied for French but it can be easily instantiated for other languages like English.
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