The aim of this study is to show how different English single content morphemes, in particular nouns and adjectives, occur in the Persian structure by applying the Matrix Language Frame and 4M models.

The data collection in the present study includes tape-recordings of spontaneous conversations involving 12 Persian–English bilingual speakers at a public university in Malaysia. The IELTS participants’ scores were 6.0 or higher and they were between 20 and 40 years old.

Qualitatively, 8 hours of tape-recorded conversations were transcribed and coded carefully according to the Canonical Trilinear Representation. Quantitatively, the English content morphemes, especially nouns and adjectives, were analysed syntactically and morphosyntactically to show how they grammatically occur in the bilingual complementiser phrases.

The findings of this study reveal that code-switching was permissible even when it led to structural dissimilarity. Wherever it was required by a Persian principle, the inserted English elements, particularly nouns and adjectives, received different Persian markers. They may also appear without any Persian marker where required by the Persian grammar. Moreover, the data supported the Matrix Language Frame and 4M models’ principles, Morpheme order principle and System morpheme principle, and no counterexample appeared against the mentioned models.

There are few studies on code-switching between Persian and English that focus on typological differences between the languages involved and the use of the Matrix Language Frame model and 4M model. Thus, the present study contributes knowledge in the field of code-switching between Persian and English and discusses how English single content morphemes, particularly nouns and adjectives, occur in the Persian structure by applying both the Matrix Language Frame model and 4M model as references.