The aim of this study is to underline the contingent problems of learners of Turkish, which is an agglutinative language and to shed light on power of mental lexicon. In this descriptive study, vocabulary challenge is investigated as it is one of the main problems for learners studying Turkish as a foreign language (TFL). This study provides information about Turkish morphology and accentuates how mental lexicon contributes foreign learners to produce Turkish words regarding form, function, and practice. Findings recommends mental lexicon be calibrated in curricula since vocabulary production is an important factor for foreign learners who come from different language families and instructional background. This study is assumed to contribute TFL teachers to have a different point of view.