This study addresses some Arabic linguistic issues that have to do with the interpretation of some grammatical structures. The researcher adopts descriptive and analytic approaches with the aim of throwing light on the concept of the Arabic infinitive known as al-Masdar al-Mutasayyad, its relationship with the phenomenon of illusion, and the discrepancy of opinions among the grammarians and exegetes regarding its grammatical analytic positions. Ultimately, the study finds that the Qur’anic verses held to have carried the infinitive, al-Mutasayyad are not exaggerated in the interpretation of their external appearance in the sentence, due to the fact that grammatical rules allow the possibility of the approximational changes which may be supported by the meaning and/or grammatical system. The findings also suggest that the Qur’anic verses which are held to have illusion are still effective to be used as evidence so as to avoid having illusion as an example of adapting evidence in the linguistics study, with reference to the occurrence of this phenomenon in the grammatical sections like exceptional word, emphasis, and conjunction. Lastly, the study shows that there is a strong relationship between the Arabic infinitive, al-Mutasayyad and the phenomenon of illusion.