時制の一致と例外

時制の一致と例外

主節の動詞が過去形のとき、that 節など従属節の動詞も過去にずらす規則を 時制の一致 と呼びます。間接話法や説明文で頻出するルールです。

基本ルール

主節が過去形 (said / thought / told / believed など) の場合、従属節の動詞も次のようにずらす：
- 現在形 → 過去形「He says he is busy. → He said he was busy.」
- 過去形 → 過去完了「He says he was sick. → He said he had been sick.」
- 現在完了 → 過去完了「He says he has finished. → He said he had finished.」
- will → would「He says he will come. → He said he would come.」
- can → could / may → might / must → had to (一部)

時制の一致の例外：
① 不変の真理 / 普遍的事実: 「She said that the Earth goes around the sun.」← 時制不一致 OK
② 現在も続く事実: 「He told me that he lives in Tokyo.」← 今も住んでいる場合 lives でOK
③ 歴史的事実: 「He said that Columbus discovered America in 1492.」← 過去形のまま
④ 仮定法: 仮定法の動詞は時制の一致を受けない「He said if he were rich, he would buy a yacht.」

注意点

① 助動詞の時制一致：
- can → could
- will → would
- may → might
- shall → should
- must → had to (義務の場合) / must のまま (推量の場合は変えないことも)
② 副詞の置きかえ：直接話法→間接話法では副詞も変える
- now → then
- today → that day
- tomorrow → the next day / the following day
- yesterday → the day before / the previous day
- this → that
- here → there
③ 「真理は現在形のまま」のルールは、学術的・科学的文章で重要。「Galileo said that the Earth moves.（動く）」← moved にする必要なし。
④ 仮定法の例外：仮定法は時制を変えない「If I were you, ...」を間接話法にしても「He said that if he were me, he would ...」と were のまま。

練習

1. 次の直接話法を間接話法に書きかえなさい：He said, "I will come tomorrow."
2. 次の文の時制の問題を判断しなさい：My teacher told us that water boils at 100℃.
3. 次の文を間接話法に書きかえなさい：She said, "I have finished my homework."