報告動詞が過去形なら、中身の時制は一段うしろへ下がります。現在→過去、過去→過去完了、will→would。ただし下がるのは基準が過去に移ったからで、移っていなければ下げません。He says he is busy. のように報告動詞が現在形なら、そのままです。過去完了はそれ以上下がりません。
When the reporting verb is in the past, the tense inside steps back one notch: present to past, past to past perfect, will to would. But it steps back because the anchor moved into the past, so when it hasn't moved, nothing shifts. "He says he is busy" keeps its present. And the past perfect has nowhere further to go.
一段下げる、ただし下げ止まりがある
One Step Back, and No Further
下げ方は決まっています。現在形は過去形へ、過去形は過去完了へ、現在完了も過去完了へ。助動詞は will → would、can → could、may → might。ここで大事なのは、過去完了より下は無いという点です。もともと過去完了だった内容は、そのまま据え置きます。同じく would / could / should / might はすでに過去形なので動きません。
The steps are fixed. Present goes to past, past goes to past perfect, and the present perfect goes to past perfect as well. Among the auxiliaries, will becomes would, can becomes could, may becomes might. What matters is that there's nothing below the past perfect. Content that was already past perfect simply stays. Likewise would, could, should, and might are past forms already and don't move.
下げる・下げ止まるStepping back, and stopping
"I work here." → He said he worked there.
「ここで働いています」→ 彼はそこで働いていると言った。
現在形が過去形へ。here も there へ。Present to past, and here to there along with it.
"I hadこの文の組み立て【過去完了】過去のある時点を基準にして、それより前に起きたことを言う形。話の中で時間を一段さかのぼるときに使う。 already left." → She said she hadこの文の組み立て【過去完了】過去のある時点を基準にして、それより前に起きたことを言う形。話の中で時間を一段さかのぼるときに使う。 already left.
「もう出ていた」→ 彼女はもう出ていたと言った。
過去完了はそのまま。これ以上は下がらない。Past perfect stays put. There's nowhere further down.
"You should rest." → Theなぜ the なのか【その場で一つに決まる】話し手と聞き手のいる状況で、どれを指すか一つに決まるから the。日本語なら「（例の）」「うちの」を補って読むと感覚が近い。 doctor said I should rest.
「休んだほうがいい」→ 医師は休むように言った。
should はすでに過去形なので動かない。Should is already a past form, so it doesn't move.
下げる(基準が過去に移った)
報告動詞が said / told のように過去形。前後関係を正確に示したいときも。The reporting verb is past, as with said or told, or you need the sequence to be exact.
She said she was living in Kobe.
彼女は神戸に住んでいると言った。
下げない(基準が動いていない)
報告動詞が現在形、または内容が今も有効なとき。The reporting verb is present, or the content still holds now.
The reporting verb is says, in the present, but the content has been stepped back anyway. The shift happens only when the anchor moves into the past. With says, the anchor is still now, so the content stays present. Writing was makes it sound like a report about a period that has already ended. The test is quick: look at the reporting verb. Said steps back; says doesn't.
as I said ...さっきも言ったとおり自分の発言を指す定型。後ろの内容は下げないことが多い。Points back at your own words, and what follows usually isn't stepped back.
he was saying ...彼が言っていたのは進行形にすると、まだ話題が続いている感じが出る。The continuous form suggests the topic is still in play.
I was told that ...〜と聞かされた受動にして出所をぼかす型。誰が言ったかを伏せられる。The passive hides the source, useful when you'd rather not name who said it.
word has it that ...噂では〜らしい出所不明の情報を伝える定型。時制は下げない。Reports something with no clear source, and the tense stays where it is.
まとめ / Summary
報告動詞が過去形なら、中身の時制を一段下げる。A past reporting verb steps the inner tense back one notch.
過去完了と would / could / should は下げ止まり。The past perfect and would, could, should have nowhere further to go.
報告動詞が現在形なら下げない。A present reporting verb calls for no shift at all.
今も有効な内容・変わらない事実は下げなくてよい。Content still true, or true regardless, can stay as it is.