そういえばCome to think of it; speaking of which
そういえば introduces a related fact or question that has just occurred to the speaker because of something said or noticed moments earlier.
How it works
The speaker first hears or notices something. That trigger brings a related point to mind, and the connective marks the moment of remembering. The new remark should be connected to the trigger, but it is usually a small associative shift rather than a simple continuation of exactly the same statement.
A related question comes to mind
来週、駅前に新しいパン屋が開くんだって。
I heard a new bakery is opening in front of the station next week.
Analyzeそういえば、駅の工事はもう終わったの？
Speaking of which, has the construction at the station finished yet?
AnalyzeThe mention of a new shop by the station reminds B of a separate but related question about the station.
A related fact is recalled
今日は山田さん、休みだね。
Yamada is off today.
Analyzeそういえば、明日から出張だと言ってたよ。
Come to think of it, Yamada said the business trip starts tomorrow.
AnalyzeThe absence makes B remember a related fact about Yamada's schedule.
Remembering versus changing the subject
そういえば says that the previous topic triggered a related memory. By contrast, ところで can simply move to an unrelated matter. Compare the relationship between the two sentences in each example.
田中さんが来月大阪へ転勤すると聞いた。そういえば、送別会はいつにする？
I heard Tanaka is being transferred to Osaka next month. Speaking of which, when should we hold the farewell party?
AnalyzeThe transfer directly prompts the question about a farewell party.
田中さんが来月大阪へ転勤すると聞いた。ところで、今日の会議は何時から？
I heard Tanaka is being transferred to Osaka next month. By the way, what time does today's meeting start?
AnalyzeThe meeting question is an independent change of subject rather than a recalled association.
Use in writing
In writing, this connective works best when the text represents a person's train of thought: a personal essay, a first-person narrative, or dialogue. In a formal report, a more direct transition is usually preferable unless the writer deliberately wants to present a personal recollection.
More examples
窓の外の桜を見て、去年の春を思い出した。そういえば、この公園でみんなと花見をした。
Seeing the cherry blossoms outside the window reminded me of last spring. Come to think of it, we all had a cherry-blossom picnic in this park.
Analyze友人が京都の話を始めた。そういえば、京都に引っ越した佐藤さんは元気だろうか。
A friend started talking about Kyoto. Come to think of it, I wonder how Sato, who moved there, is doing.
Analyzeカレンダーの連休を見て気づいた。そういえば、ホテルをまだ予約していない。
Seeing the long weekend on the calendar made me realize something. Come to think of it, I still haven't booked a hotel.
Analyze妹が新しいスマートフォンを買ったと聞いた。そういえば、私の契約も来月で切れる。
I heard that my younger sister bought a new smartphone. Come to think of it, my contract expires next month too.
Analyze同僚が駅前の喫茶店の話をした。そういえば、あの店に傘を忘れたままだ。
A coworker mentioned the coffee shop by the station. Come to think of it, I still haven't retrieved the umbrella I left there.
Analyze古い写真を見て、先生のことを思い出した。そういえば、先生は今どこで教えているのだろう。
An old photograph reminded me of my teacher. Come to think of it, I wonder where the teacher is teaching now.
Analyze