The knowledge of spelling cannot be earned by learning the relations between the sounds and graphemes. However, this study shows that the learners' knowledge is not a mere result of simple memorization. The current study found that the learners’ spelling knowledge is interwoven with their established phonemic inventory and their imperfect knowledge of the target language spelling. Using a learner corpus (KNU English Learner Corpus) constructed from young learners, primary school grades from 4 to 6, enrolled in a gifted English program, we extracted and examined 125 erroneously spelled words each from the basic level and the advanced level subcorpus. Building on Justicia et al. (1999), the error data were classified into six categories and an additional category of morpheme error (MOR) with substitution and omission errors accounting for 76.3% of the total errors. Conducting further analysis, we were able to identify typical cases of the errors and the underlying causes inducing the errors. Generally speaking, the substitution error is found to be due to the variability between grapheme and sound whereas the omission error is often caused by the learners’ tendency to prefer simpler graphemephoneme relations. There also exists considerable intervention of the learners' Korean phonemic inventory, particularly in lower level learners. The findings from this study can be used to provide various specific guides for effective spelling instruction and help learners become more confident about their writing.