Several studies suggest that third language acquisition (TLA) is marked with complex patterns of language interaction. However, it is not clear yet to what extent multilinguals activate each of their background languages in TLA, as various factors may trigger the activation of one of the previously learnt languages. This study aims to contribute to the discussion by examining the use of verbal morphology in third language (L3) Turkish of Russian–English–Turkish trilinguals. We investigate whether the use of verbal morphology in L3 Turkish of Russian–English–Turkish trilinguals differs from that of Turkish native speakers and in the case of a deviation, which of the background languages can account for it.

The study is done within the framework of cross-linguistic influence.

The data are collected from eight native speakers of Russian who are highly proficient in their L2 English and L3 Turkish, and use their three languages more or less equally every day. Fictional narratives are employed in the study as the tool for data collection. The use of finite and non-finite verbal forms in Turkish is compared with the baseline data coming from native speakers of Turkish.

The results reveal that the trilingual participants mainly use the verbal morphology consistently with native speakers of Turkish; nonetheless, two deviations from the Turkish baseline stand out: tendency to use finite verbal forms in their noun clauses and inconsistency in the use of temporal-aspectual markers in finite verbal forms. The former can be attributed to the L1 and L2 interlanguage, while the latter can be attributed to the influence of particularly L1 Russian.

Thus, the study provides evidence that the source of interference into L3 production is likely to occur due to the cross-linguistic influence from L1, which in turn might imply the predominant position of L1 over sequentially learnt languages at the morphosyntactic level in the language processing.