One study showed that Czech and Slovak lexicons differed by 80 percent, but this high percentage was found to stem primarily from differing orthographies and slight inconsistencies in morphological formation; Slovak morphology is more regular (when changing from the nominative to the locative case, Praha becomes Praze in Czech and Prahe in Slovak). The two lexicons are generally considered similar, with most differences in colloquial vocabulary and some scientific terminology. Slovak has slightly more borrowed words than Czech.
Try to answer this question if possible (otherwise reply "unanswerable"): When is Slovak morphology more regular than Czech?
when changing from the nominative to the locative case