In this paper I will discuss the formation of different types of yes/no questions in Serbian (examples in (1)), focusing on the syntactically and semantically puzzling example (1d), which involves the negative auxiliary inversion. Although there is a negative marker on the fronted auxiliary, the construction does not involve sentential negation. This coincides with the fact that the negative quantifying NPIs cannot be licensed. The question formation and sentential negation have similar syntactic effects cross-linguistically. This has led to various attempts to formulate a unifying syntactic account of the phenomena (ever since Klima 1964). One striking fact about the two syntactic contexts is that both license weak NPIs (Negative Polarity Items). It has been suggested (cf. Laka 1990, Culicover 1991) that the derivation of both interrogatives and negatives involves the same type of functional projection PolP (polarity phrase). One such account of the formation of negative interrogatives in Serbo- Croatian is offered by Progovac (2005). She proposes that there are two PolPs optionally cooccurring in the same clause, in which both positive and negative polarity items check their positive or negative features (following Haegeman and Zanuttini (1991) feature-checking account of negative structures, and the insights of Brown(1999) on the negation in Russian). On her account, the negative auxiliary question in (1d), is the case when both polarity phrases are present. The higher has [-pos +neg] features, and the lower one (below TP) is [-pos -neg]. Although her account correctly predicts the ungrammaticality of (2a) in contrast with (1c), it wrongly predicts the (2b) to be grammatical. I will argue that Progovac’s theory regarding the nature of the PolP is wrong. It employs both the binary feature valuation on the polarity head and the hierarchical ordering of the two polarity phrases, which eventually leads to overgeneration. On the account presented here the nature of the question marker (li vs zar) is highly relevant. Notice that (1b) and (1d) express presuppositions regarding the truth value of the propositions. In this way they contrast with (1a) and (1c). In addition, the type (1b) (with the question particle zar) can introduce both the positive and negative presupposition as shown in (3), which, semantically, makes this construction compatible with negative auxiliary questions in English (4a). The polarity items licensed in the relevant structures are also of the same type in both languages. The fronted-negative-auxiliary questions (1d) in Serbian are only possible with the particle li. In this case the presupposition is exclusively positive. The peculiar question/focus marking function of li (in Bulgarian and Russian) is well known. However, it is always assumed that its focus marking role is not relevant for the formation of yes/no questions. This I believe is not correct. The syntactic explanation of the interpretational facts points to the following: A) The possibility of the separate lexical encoding (particle zar) of the ‘rhetorical’ yes/no questions in Serbian allows the embedding of both positive and negated sentences, in which case the (weak) NPIs can remain in local relation with the negated verb. B) Recall that Serbian is an NC language, which requires local/c-command relation between the verbal negative marker and the NPI. With the negative inverted auxiliary questions this condition is not met, and the licensing of an n-word is not possible. C) The impossibility of licensing a weak NPI (i-words in the examples below) is due to the nature of the question marker li. (1) a. Da li je Vera videla ikoga / nekoga / *nikoga? DA Q aux Vera see.part.F.Sg anyone someone noone “Did Vera see anyone/someone/noone?” b. Zar je Vera videla ikoga / nekoga / *nikoga? ZAR aux Vera see.part.F.Sg anyone someone noone “Is it really the fact that Vera saw anyone/someone?” c. Je li Vera videla ikoga / nekoga /*nikoga? aux Q Vera see.part.F.Sg anyone someone noone “Did Vera see anyone/someone/noone?” d. Nije li Vera videla *ikoga / nekoga / *nikoga? neg+aux Q Vera see.part.F.Sg anyone someone noone “Didn’t Vera see someone?”/ “Vera saw someone, didn’t she?” (2) a. *Nije li Vera videla nikoga? neg+aux Q Vera see.part.F.Sg noone b. *Nije li Vera videla ikoga? neg+aux Q Vera see.part.F.Sg anyone (3) a. Zar je Vera videla nekoga / ikoga? ZAR aux Vera see.part.F.Sg someone/anyone b. Zar Vera nije videla nekoga/nikoga? ZAR Vera neg+aux see.part.F.Sg someone/anyone (4) a. Didn’t Vera (NOT) see someone/anyone? b. Vera saw someone, didn’t she?