Purposeful and strategic use of L1 can help with L2 learning. However, in many contexts, monolingual immersion approaches dominate, leading language teachers to refrain from using L1. It can also mean that teachers are not professionally prepared to implement evidence-informed uses of L1. In this article, we share the findings of an intervention study that aimed to raise preservice English language teachers' awareness of purposeful L1 use while co-exploring ways generative artificial intelligence (AI) tools (e.g. ChatGPT) can aid teachers' knowledge development and strategic utilization of L1 in L2 classrooms. Data were collected from fifty-six preservice language teachers in Hong Kong through a pre- and post-intervention mixed-method survey and follow-up group interviews. The findings show that explicit instruction on the use of L1 in L2 classrooms can increase preservice teachers' intention to use L1 as well as their knowledge about the evidence-informed use of L1 and the ways in which generative AI tools can assist their implementation of L1.