a

He is not a teacher.

【Translation】He is not a teacher. 【Explanation】Negative sentences with be-verbs are structured as [Subject (He) + be-verb (is) + not + Complement (a teacher)]. The correct order is 'He is not a teacher.' The 3rd word (★) is 'a'.

【和訳】彼は先生ではありません。 【解説】be動詞の否定文は【主語 (He) ＋ be動詞 (is) ＋ not ＋ 補語 (a teacher)】の語順になります。正しい文は「He is not a teacher.」となり、3番目の空欄（★）には「a」が入ります。