Since the 1970s, findings from Morpheme Order Studies (MOS) have suggested that the emergence of morphemes follows a predictable order in L2 English. This paper shows how the tools and practices in Learner Corpus Research (LCR) offer a richer descriptive basis, which is achieved with Interlanguage Annotation (ILA), a manual, fined-grained, purpose-oriented type of annotation. Additionally, the paper uses a standardised placement test, since proficiency level has been overlooked in most previous MOS. Both of these practices provide a more detailed description of morpheme accuracy order across different levels. The paper analyses four proficiency levels (A1-B2) in a subcorpus of L1 Spanish-L2 English secondary-school learners from the CORpus of English as a Foreign Language (COREFL). The results partially confirm findings from previous MOS, but also reveal key findings that had gone previously unnoticed regarding the role of proficiency level and the subtype of errors, which are relevant factors for SLA research.