Previous neuroimaging studies have demonstrated that the left inferior frontal gyrus (IFG), the brain region known as Broca’s area, is activated during sentence comprehension and sentence production. However, as previous studies have only tested the correlation between left IFG activation and sentence processing, the causal relationship has remained unclear. Using electroencephalography (EEG) and transcranial electrical stimulation (tES), a non-invasive brain stimulation technique that modulates brain activation, we examine whether higher activation in the left IFG improves sentence comprehension in a native language (Experiment 1) and a non-native language (Experiment 2). In Experiment 1, the participants performed a sentence comprehension task using two types of Japanese sentences (active and passive sentences). They showed shorter reaction times for syntactically more complex passive sentences but not for active ones. In Experiment 2, the participants who were naïve to Spanish learned verb conjugations in Spanish during tES and performed verb conjugation and short-term memory tasks. We found higher task accuracy and shorter reaction times on the verb conjugation tasks but not on the short-term memory task. Moreover, the tES group showed the left anterior negativity, an event-related potential (ERP) typically observed when native speakers make syntactic judgments, while the sham group showed N400, an ERP typically observed when non-native speakers make syntactic decisions. To further clarify the causal relationship between neural oscillations and sentence comprehension, we examined whether modulating neural oscillations by tES can change the interpretations of structurally ambiguous sentences that can have two distinct meanings (Otoko-o nagutta, Onna-o ketta.; Someone hit the man and kicked the woman or someone kicked the woman who hit the man) (Experiment 3). We found that the ratio between two sentence comprehensions changed after tES, suggesting that tES modulated comprehensions of structurally ambiguous sentences. These results elucidate a causal relationship between the left IFG and language processing.