on

I play tennis on Sunday.

【Translation】I play tennis on Sunday. 【Explanation】The standard English sentence structure is [Subject (I) + Verb (play) + Object (tennis) + Time (on Sunday)]. Remember that we use the preposition 'on' before days of the week. The correct order is 'I play tennis on Sunday.' The 3rd word (★) is 'on'.

【和訳】私は日曜日にテニスをします。 【解説】英語の語順ルールは【主語 (I) ＋ 動詞 (play) ＋ 目的語 (tennis) ＋ 時を表す表現 (on Sunday)】となります。「on Sunday」のように、曜日の前には前置詞「on」を置きます。正しい文は「I play tennis on Sunday.」となり、3番目の空欄（★）には「on」が入ります。