The difference between 時　and 時は

Question

I know that it's correct to use は　after 時, and it's also correct to omit it. For example:

子供の時、よく海で泳ぎました。

子供の時は、よく海で泳ぎました。

How does the meaning of the sentence change when the particle is used and when it's not? This may be a slight change, but still.

Very similar to a question you asked last month: japanese.stackexchange.com/questions/65466/… — BJCUAI, Mar 13, 2019 at 3:06
@BJCUAI Thank you! So, it's the same thing then... I've just come across the following sentence: 学生の時にもっと勉強しておけばよかったですよ。Here they use ni after toki. Is is the same as ... toki wa...? — Enguroo, Mar 13, 2019 at 3:10
Roughly similar to the difference between 'In my childhood' vs. 'As for my childhood'. As with most things it is not directly equivalent, but I think you get the nuance. — BJCUAI, Mar 13, 2019 at 3:27
学生の時にもっと勉強しておけばよかったですよ。Here they use ni after toki. Is is the same as ... toki wa...? >> 「学生の時はもっと勉強しておけばよかった」 sounds unnatural. — chocolate, Mar 13, 2019 at 3:33
No... You say 「～～ときに(or～～とき)…ておけばよかった」"I should have … / I regret that I didn't … when ~~ " eg 「安いときに買っておけばよかった」"I should have bought it when it was cheaper" (← In fact, I did NOT buy it.) but not 「安いときは /安いときには買っておけばよかった」. If you mean "I needed to study more when I was a student, but now I don't need to study hard anymore." (← In fact, I DID study.), you can say 「学生の時は、もっと勉強しないといけなかった。/勉強しなければならなかった。」 — chocolate, Mar 13, 2019 at 15:45

chocolate · Accepted Answer · 2019-03-13 03:58:42Z

子供の時、よく海で泳ぎました。
子供の時は、よく海で泳ぎました。

You might say the latter to imply...

"I used to swim in the sea in my childhood (but now I don't anymore)."

Here, the は is functioning as the contrastive particle (対比の「は」).

You might also say the latter when responding to a question...

"What did you often do in your childhood?"
-- "In my childhood, I often swam in the sea."

Here, the は is functioning as the topic particle (主題の「は」).

You'd say the former to just neutrally state that you would often swim in the sea in your childhood.

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