？
In the past subjunctive, all be-verbs become 'were,' you know?
Literal
Subjunctive-past [of-の] case be-verb [topic-は] all 'were' [into-に] become [explanatory-んだ][emphatic-よ]?
Educational register discussing English grammar in Japanese. 仮定法過去 is the Japanese translation of 'past subjunctive' in English grammar. The sentence uses full-width Latin characters (ｂｅ動詞, ｗｅｒｅ) which is characteristic of Japanese text where Latin letters get 'Japanized' width. んだよ with a rising tone functions as 'you know?' — checking that the listener has caught the point. A tutor explaining a grammar rule.