Some subordinating conjunctions reveal whether speakers believe the propositions the conjunctions connect and how speakers believe propositions are connected. Because and although express belief; if + indicative and unless uncertainty, and if + subjunctive, disbelief. Because, if + indicative and if + subjunctive express a positive entailment relation between clauses while although and unless express negative entailment. To validate this analysis a comprehension task was administered to 96 subjects divided among 9-, 11- and 19-year-olds. Nine-year-olds understood although and because while adults also comprehended unless and if + indicative. Generally the more positive beliefs the conjunction embodied, the easier its comprehension. The order of difficulty was because, although, if + indicative, unless, and if + subjunctive. Older subjects more often confused if + subjunctive with if + indicative. Understanding of the uncertainty conveyed by if + indicative and unless was linked to performance on a task of detecting uncertain outcomes.