What's the difference between こそ and さえ in 「忙しくて昼ごはんを食べる時間＿＿＿ない」


I don't know which to choose:

> 忙しくて昼ごはんを食べる時間 **さえ** ない

or

> 忙しくて昼ごはんを食べる時間 **こそ** ない

The explanation in my textbook is given in such a way that I can't understand
the difference:

It says こそ "expresses emphasis" and for さえ it gives an example "Even children
know that" with no additional elaboration and that example is also, I think,
emphasis, so how to understand?




The main difference is that こそ is used to single out something as a primary
"example" of something. It is usually only used to emphasize something
"positive". It most often will replace は to add the emphasis. It's not a
direct translation, but it might help to think of it in term of something like
"especially" or "particularly".

>   * 音楽 **こそ** 命だ → _Music_ is life
>   * これ **こそ** 待っていたものです → _This_ is what I've been waiting for (more than
> anything else)!
>   * 今 **こそ** チャンスなんだ！ → _Now_ (more than any other time) is our chance!
>   * 人間関係に **こそ** 人生の価値がある → It's (particularly/especially) _in
> relationships_ that one's life has value!
>

さえ singles out an "extreme", implying that the statement obviously applies to
other examples. It most often carries a negative connotation, but not always.
It is usually replacing も or を in the sentence, and can be thought of as
"even". Actually, I believe every occurrence of さえ can be replaced by も, but
not necessarily the other way around. Here are some of my examples from
[another さえ post](https://japanese.stackexchange.com/q/4864/78) that you can
refer to for more info:

>   * 先週の病気は本当に辛【つら】かった。水 **さえ** 飲めなかった。 → "My illness last week was really
> bad. I couldn't **even** drink water!" → When one is sick, you expect them
> not to be able to drink alcohol, soda, etc. But water should be OK for most
> people. **Even water** , the most basic thing was intolerable for this
> person.
>   * えりこは親友の花子 **にさえ** 知らせずに外国へ旅立った。 → "Eriko left on a trip to a foreign
> country without telling **even** her best friend Hanako." → You'd expect her
> to not tell her co-workers, acquaintances, etc. But best friends usually
> tell each other everything, and she didn't tell **even** her.
>   * 山の上には夏 **でさえ** 雪が残っている。 → " **Even** if it's summer, there is snow left
> on the top of the mountain." → You expect snow to be on top of a mountain in
> winter, and some leftover in the spring. But usually by summer it's all
> gone, but for this mountain the snow remains **even** then.
>

So for your sentence, さえ is the correct choice. It sounds ungrammatical to me
that have こそ in there. But regardless, さえ is needed for the overall meaning.

>   * 忙しくて昼ごはんを食べる時間 **さえ** ない → I'm so busy, I don't **even** have time to
> eat lunch → If you're busy, of course you don't have time to go shopping, to
> the movies, etc. But this person is so busy, there's not even enough time
> for them to eat. Of course, this may be an exaggeration, but it still
> emphasizes just how busy they are.
>   * それが子供に **さえ** 分かる常識です → That's such common sense that **even** children
> know it.
>




The main difference is that こそ is used to single out something as a primary
"example" of something. It is usually only used to emphasize something
"positive". It most often will replace は to add the emphasis. It's not a
direct translation, but it might help to think of it in term of something like
"especially" or "particularly".

>   * 音楽 **こそ** 命だ → _Music_ is life
>   * これ **こそ** 待っていたものです → _This_ is what I've been waiting for (more than
> anything else)!
>   * 今 **こそ** チャンスなんだ！ → _Now_ (more than any other time) is our chance!
>   * 人間関係に **こそ** 人生の価値がある → It's (particularly/especially) _in
> relationships_ that one's life has value!
>

さえ singles out an "extreme", implying that the statement obviously applies to
other examples. It most often carries a negative connotation, but not always.
It is usually replacing も or を in the sentence, and can be thought of as
"even". Actually, I believe every occurrence of さえ can be replaced by も, but
not necessarily the other way around. Here are some of my examples from
[another さえ post](https://japanese.stackexchange.com/q/4864/78) that you can
refer to for more info:

>   * 先週の病気は本当に辛【つら】かった。水 **さえ** 飲めなかった。 → "My illness last week was really
> bad. I couldn't **even** drink water!" → When one is sick, you expect them
> not to be able to drink alcohol, soda, etc. But water should be OK for most
> people. **Even water** , the most basic thing was intolerable for this
> person.
>   * えりこは親友の花子 **にさえ** 知らせずに外国へ旅立った。 → "Eriko left on a trip to a foreign
> country without telling **even** her best friend Hanako." → You'd expect her
> to not tell her co-workers, acquaintances, etc. But best friends usually
> tell each other everything, and she didn't tell **even** her.
>   * 山の上には夏 **でさえ** 雪が残っている。 → " **Even** if it's summer, there is snow left
> on the top of the mountain." → You expect snow to be on top of a mountain in
> winter, and some leftover in the spring. But usually by summer it's all
> gone, but for this mountain the snow remains **even** then.
>

So for your sentence, さえ is the correct choice. It sounds ungrammatical to me
that have こそ in there. But regardless, さえ is needed for the overall meaning.

>   * 忙しくて昼ごはんを食べる時間 **さえ** ない → I'm so busy, I don't **even** have time to
> eat lunch → If you're busy, of course you don't have time to go shopping, to
> the movies, etc. But this person is so busy, there's not even enough time
> for them to eat. Of course, this may be an exaggeration, but it still
> emphasizes just how busy they are.
>   * それが子供に **さえ** 分かる常識です → That's such common sense that **even** children
> know it.
>

