<p> It has been reported that Japanese learners of English (JLEs) regard that The door opened is ungrammaticaland should be revised to the passive voice:The door was opened (Otaki &Shirahata, 2017). It seems that these learners think that we should use the passive voice instead of the active voice when the subject of the sentence is an inanimate noun such as the door. In this paper, we explore why this phenomenon happens. We first discuss this issue by observing previous acquisition studies of English ergative verb usages by adult JLEs, and confirm the claim that their use of the passive voice in intransitive ergative verb sentences is indeed due to an inanimate subject. Then, the study insists that this animacy issue of the subject is not due to the learners' L1, Japanese, but to a 'fundamental language learning strategy': The first noun of the sentence should be Agent, which is called the Agent First principle (Jackendoff, 2002). Moreover, we conducted a longitudinal experiment and claim that explicit grammar instruction can be effective to promote JLEs' comprehension of English ergative verb usages at least for a certain period of time.</p>