Should the mother tongue be used in language teaching?

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Hi!

I teach English as a second language to high school students and I have always thought that I should use the English language only whenever I'm inside the classroom. However, I have recently read a study that says explanations should be in the mother tongue. I suppose this is true for beginners but how about for those who already have a basic knowledge on grammar?

Thank you very much for responding to this.

~ John Lour Flores :)