This paper investigates the interlanguage grammar of a child second language (cL2), and argues for the Missing Surface Inflection Hypothesis (MSIH). It analyzes a novel longitudinal case study documenting a native Arabic-speaking child acquiring English naturalistically from age four. The findings reveal a clear dissociation between the learner’s highly variable use of inflectional morphology and her otherwise robust syntactic competence. Despite inconsistent verbal inflections, the learner consistently demonstrated high accuracy in complex syntactic domains, including null subjects, case assignment, and English-specific question formation. Notably, the data show a lack of observable negative transfer in production from her L1 (Arabic), despite major typological differences, a finding that challenges strong L1 determinism in early cL2 acquisition. The learner’s error patterns align with Grammatical Conservatism, where morphological omissions co-occur with syntactic accuracy. These results validate the MSIH in a previously undocumented L1-L2 pairing and underscore the distinction between core computation and its surface realization in second language acquisition.

1. INTRODUCTION
2. MORPHOLOGICAL VARIABILITY IN SLA
3. THEORETICAL FRAMEWORKS
    3.1. The Null Subject in Principles and Parameters Theory
    3.2. The Null Subject in the Minimalist Program
    3.3. The Null Subject Parameter in Second Language Acquisition
    3.4. Case Assignment: From Checking to Valuation
    3.5. Question Formation: Syntactic Operations in English and Arabic
4. DATA
    4.1. Verbal Morphology
    4.2. Null Subjects
    4.3. Question Formation
    4.4. Case Assignment
5. DISCUSSION
    5.1. Evidence of Complex Syntactic Competence
    5.2. Null Subjects
    5.3. Case Assignment
    5.4. Question Formation
    5.5. L1 Influence and the Debate on Syntactic Transfer
6. CONCLUSION
REFERENCES

• Abdulaziz Alnajmi(Assistant Professor, Department of English, Umm Al-Qura University, Taif Road, Makkah, 24382, Saudi Arabia)