Vowel Development in Korean-Speaking Children Aged 18–35 Months at the Word Level

Objectives: Vowels are generally acquired earlier and are typically produced with greater accuracy than consonants, yet their developmental trajectories in young Korean children remain underexplored. This study investigated early vowel development in Korean-speaking children aged 18 to 35 months, focusing on vowel accuracy, error patterns, and their relationship with receptive and expressive language abilities across both picture-naming of meaningful words and a nonword repetition (NWR) task.Methods: Sixty-eight typically developing children participated, divided into three age groups: 18–23 months, 24–29 months, and 30–35 months. Children completed two production tasks: a picture-naming task including meaningful words and an NWR task, each incorporating seven Korean monophthongs (/a, i, u, ɯ, o, ʌ, ɛ/). Productions were analyzed for vowel accuracy and categorized into four error types: omission, substitution, vowel harmony, and diphthongization. Substitution errors were further classified by vowel height, front–back, and rounding. Receptive and expressive language abilities were measured using the Sequenced Language Scale for Infants.Results: Vowel accuracy increased significantly with age in both word and nonword conditions, and was consistently higher in words than in nonwords. Omission errors decreased with age, particularly in nonwords, whereas substitution, vowel harmony, and diphthongization occurred at low and relatively stable rates; in the oldest group (30–35 months), omission errors were rare, while vowel harmony errors became relatively more frequent. Substitution errors were most frequently related to vowel height. Receptive and expressive language abilities were positively correlated with vowel accuracy in both task conditions, and both were negatively correlated with omission errors, especially in nonwords, whereas other error types showed limited associations with language scores.Conclusion: These findings suggest that vowel accuracy improves during the second and third years of life in both word and nonword contexts, and that a developmental shift from syllable-level omission toward more segmental processes such as vowel harmony reflects reorganization and maturation rather than regression. Vowel analysis provides meaningful insights into early phonological development and its association with receptive and expressive language growth in Korean-speaking toddlers.