This work explains the types of affixation errors second language learners make when learning English word formation processes, especially derivational and inflectional affixations. The data for the study were collected as primary sources from two secondary schools in Sweden. The data were analyzed with the use of Error Analysis noted by Corder (1967) and the error analysis framework adapted by Ellis et al. (2005, p. 57). The method chosen was to identify, classify, describe,and evaluate derivational and inflectional affixation errors. In total 2,812 answers were retrieved. The results consist of some findings, for example, some of the derivationaland inflectional affixations errors were noticed to be intralingual and interlingual. Also, the nature of the errors issuch that they are either transferred, omissive, additive or substitutive errors. Moreover, the errors were also due to overgeneralization, including substitutionerrors, or additive errors. Previous research findings showedstudents make grammatical errors with letter insertions, letter omission, or substitutionerrors. This study made the same findings as students made errors of letter insertion, letter omission, substitution errors, and errors due to overgeneralization. Some of the most difficult derivational and inflectional affixation errors were also noticed across all the grades.