The purpose of this study is to investigate Japanese learners' orthographic knowledge of English derivational morphology. When suffixes are attached to stems in order to form derived words, orthographical changes are often caused. For example, the final e is deleted when -ity is attached to active to form activity; similarly, o in long is changed to e when -th is attached to long to form length, reflecting not only orthographical but underlying phonological and morphological change represented in orthography. Although this type of orthographic knowledge is important for the formation of orthographically appropriate derived words, there have been only a few studies of this problem in first languages, and none, to our knowledge, in second languages.
Orthographic knowledge of English derivational morphology among Japanese learners of English was examined using a task requiring the production of a derived word to finish a sentence. The result showed that the more proficient the learners were, the more accurate their orthographic knowledge of English derivational morphology. However, it was also found that while the learners had accurate knowledge of relatively simple orthographic rules (e.g., active → activity), their accuracy dropped significantly when relatively complex rules requiring both orthographic and phonological knowledge (e.g., long → length) were tested. An error analysis showed that the most frequent error type across all proficiency levels was attaching a suffix to a stem without an orthographic change, leading to errors even where the suffix was correct (e.g., longth). In conclusion, while orthographic knowledge becomes more refined as learners become more proficient, some types of orthographic knowledge are still at the developmental stage even for advanced learners. Finally, further findings from the results are discussed, and future research directions suggested.