The objective of this study was to assess the phonetic use of the allomorphic variations of the past [+ed] and plural [+s] morphemes by a sample of 45 EFL semester one students at Ibn Tofail University, Morocco, using error analysis approach. To collect relevant data, two written tests were designed and distributed to a randomly selected sample to be filled in separately during 40 minutes: the first test consisted of a set of past regular verbs including some adjectives ending in ed, and the second one included a number of English nouns requiring the regular plural [+s]. Further, the participants were given an adapted list of nonce words from Berko’s study (1985). The principle was if they could provide the plural marking of the unfamiliar words correctly, it would be concluded that they knew how to pluralize words in different phonological contexts. The findings of the study demonstrated that the majority of the subjects failed to use the allomorphic variations /t/ and /id/ in different phonetic environments, substituted the latter allomorphs with the allomorph /d/, and erroneously generalized the allomorph /d/ to the adjectives that end in ed. In the second test, a great number of the subjects performed better in pluralizing the nouns that require the voiceless alternant /s/, but found the allomorph /iz/ more complex than the others since it requires a vowel insertion rule to break the cluster of two consonants having similar point of articulation, and overgeneralized the allomorph /-z/ to the nonce words requiring /-iz/.