時制の一致

時制の一致 ("agreement of tenses") is the term Japanese school grammar uses for English tense backshifting — and the point is that Japanese has no such rule. English shifts the tense of a reported clause to match the reporting verb: "I'm tired" becomes "She said she was tired." Japanese keeps the tense the original speaker or thinker used: 彼女は疲れていると言った — literally "she said she is tired."

The reason is that Japanese subordinate tense is relative, not absolute. Inside an embedded clause, the dictionary (る) form means "not yet completed at the time of the main event" and た means "already completed at the time of the main event" — both measured from the main clause, not from the moment of speaking:
• Simultaneous → る / ている: 雨が降っていると思った — "I thought it was raining."
• Later → る: 明日来ると言った — "[They] said [they] would come tomorrow."
• Earlier → た: もう食べたと言った — "[They] said [they] had already eaten."

The same relative logic governs adverbial clauses, where English does not backshift either but Japanese meaning changes with the verb form:
• 日本に行くとき、カメラを買った — the purchase came before arriving in Japan (still "going" at that point).
• 日本に行ったとき、カメラを買った — the purchase came after arriving (the going was complete).
前に always takes the non-past (行く前に) and 後で always takes た (行った後で), for the same reason.

Backshifting a Japanese reported clause the English way changes its meaning: 疲れていたと言った means "said [they] had been tired" — tired at some point before the statement. Learners translating from English should therefore keep the original tense inside と言う / と思う / と聞く clauses, and English speakers reading Japanese should supply the backshift only in translation.