Abstract
The paper provides experimental evidence (naturalness rating paradigm) that wh-extraction from clausal adjuncts (conditionals, temporal clauses, purpose clauses) is grammatical in Czech, although it is fully natural only in one specific condition: if the adjunct is left-peripheral (vs. right-peripheral/central), if the adjunct is not headed by nominal material (the case if..., the moment when...), and if the extracted wh-word is a relative pronoun (vs. an interrogative phrase). The evidence supports the analysis proposed in Biskup & Šimík (2019; https://ling.auf.net/lingbuzz/004573), whereby left adjuncts are proposition-denoting clauses/CPs (and hence transparent), while right adjuncts are entity-denoting free relatives/DPs (and hence islands).
The evidence is incompatible with a discourse-based analysis of islands (e.g. Abeille et al. 2020; https://doi.org/10.1016/j.cognition.2020.104293), which predicts right (focused) material to be more transparent than left (backgrounded) material.