The article provides a methodological discussion on the definition of linguistic structures in interlanguage. Data come from an Eritrean learner observed during the first months of exposure to Italian as a second language. Previous research identified some developmental sequences for the acquisition of verb morphology, but it is not always clear how metalinguistic terms used in such sequences (e.g. ‘present tense’, ‘past participle’) should be interpreted. The article discusses
the issue in depth, by providing several explicit definitions for some of the structures involved in the sequence, notably present tense, person marking, past participle and present perfect. Results show that different definitions of the interlanguage structures may lead to different acquisition orders. These findings suggest that the formulation of developmental sequences should be based on clear, explicit definitions both of the acquisition criteria and of the structures investigated.