The present work is an attempt to explain how the structure of derived words influences the order of acquisition of word formation rules. Three main types of derivatives — transpositions, modifications and mutations — are investigated in Polish child language. The differences among the three categories (analyzed in nouns, verbs and adjectives) are discussed, regarding semantic and formal changes undergone in the process of forming complex words. It is claimed that semantic-formal correspondence is an essential factor influencing the order of acquisition of word formation rules. Modifications in which the correspondence is both preserved and has a simple character emerge earlier than the other types of derivatives — transpositions (the correspondence is broken) and mutations (the correspondence is complex). The proposal is said to be complementary to Clark’s principles of acquisition of complex words (Clark, 1993). Polish has a very rich word formation system (compared e.g. with English), thus the analysis of word formation acquisition in such a language seems to be especially important for developmental psycholinguistic research. The proposal is tested on four Polish children’s speech diaries available in CHILDES (Smoczynska, 1998). All data available from ages two to seven were analyzed. The analyses revealed that indeed modifications are acquired earlier than transpositions and mutations. The consequences of the finding for psycholinguistic theory are discussed.