Should English teachers use only English in class? Many people think so. But some studies say a little Japanese, or another first language, can help. In a Korean study, children and adults learned new English words better when the teacher explained them quickly in Korean. The effect was bigger for children.

However, too much first language means less English practice. So teachers should use it for a short time and for a clear reason.

In Japan, high school English classes are expected to be taught mainly in English. The goal makes sense, because students have few chances to use English outside school. But research suggests that a little first-language use can help. A Korean study compared English-only explanations with brief explanations in Korean for 443 elementary school children and 286 university students. Both groups learned more new words when Korean was used, and the children benefited more.

A 2026 meta-analysis of 24 studies also found an advantage for approaches that use the first language, especially for beginners. However, a review found that many such studies had a high risk of bias. The best approach may be to use the first language briefly for meaning and English for practice.

For decades, many language teaching experts have argued that classes should be conducted entirely in the target language, and Japan's high school curriculum now states that English lessons should, in principle, be taught in English. The reasoning is that learners in a foreign-language setting receive little input outside the classroom. Empirical studies, however, give a more mixed answer.

In a large Korean study, both 443 primary school children and 286 university students learned more vocabulary when teachers briefly used Korean to explain new English words, and the advantage was greater for the younger learners. A study of five- and six-year-olds reported a similar benefit that was still visible two weeks later. A 2026 meta-analysis of 24 studies estimated a moderate advantage for multilingual approaches, particularly for beginners.

But the evidence has weaknesses. A 2023 systematic review found only ten eligible studies, and four of the five that favored translanguaging were judged to have a high risk of bias. A Spanish study, by contrast, found that vocabulary was learned better through English-only teaching. The sensible conclusion is not that teachers should switch freely between languages, but that brief, purposeful first-language use can save time without sacrificing valuable English input.