Learn Hardcore Japanese: kanozyo no hyouzyou wo mireba, de genki ga nai to wakaru. - If you look at her expression, you can tell at a glance that she isn’t well.

彼女の表情を見れば = If you look at her facial expression (you can read her mood/emotion).
彼女の顔を見れば = If you look at her face (more neutral; context might still imply expression, but it’s less precise).

Using 表情 focuses on interpreting how she feels from her expression, which matches the idea of “you can tell she’s not well.”

Why is の used in 彼女の表情? Could you say 彼女は表情 instead?

の here marks possession: 彼女の表情 = her expression.

彼女の表情: “the expression of hers / her expression” (a noun phrase)
彼女は表情…: sounds like “as for her, (her) expression …” and would need to continue as a full sentence. By itself, 彼女は表情を見れば does not work, because 表情を見れば already has a subject (implicit “you / someone”) and 彼女は would become a confusing extra topic.

You need の because you’re describing whose expression it is: her expression, not “as for her, expression …”.

Why does 表情 take the particle を in 表情を見れば?

を marks the direct object of the verb.

Verb: 見る = to look at / see
Object: 彼女の表情 = her expression

So:

彼女の表情を見る = to look at her expression
With ～ば: 彼女の表情を見れば = If (you) look at her expression.

You cannot use が here because 彼女の表情 is not the subject doing the action; it is the thing being looked at. The (implied) subject is you / one / people doing the looking.

What does the ～れば form in 見れば mean? How is it different from 見ると or 見たら?

見れば is the conditional ～ば form of 見る.

In this sentence, it means roughly “if you look (at)” or “when you look (at)” and often has a nuance like “from the point of view of looking…” or “judging by looking…”.

Comparisons:

彼女の表情を見れば、…
If you look at her expression (judging from it), …
→ Slightly more formal / written, can sound like “from that standpoint”.
彼女の表情を見ると、…
When(ever) you look at her expression, …
→ と often implies a natural, automatic result.
彼女の表情を見たら、…
When you look at her expression / If you happen to look, …
→ More casual, often one-time or situational.

All three could work in similar contexts, but 見れば feels a bit more like “if you look / judging from looking” in a general, somewhat objective way.

What is the role of 一目で? Why does it use で?

一目 (ひとめ) means one look / one glance.

The pattern Xで often means “with X / by X / in X” and functions adverbially.

一目で = with one look / at a glance / by just one glance

So:

一目で分かる = to know/see/understand at a glance

In this sentence:

一目で元気がないと分かる
= you can tell at a glance that she isn’t energetic / well.

で is turning 一目 into something like a manner/means adverb: “by a single glance.”

Why do we say 元気がない instead of using 元気をない or just 元気ない?

元気がある / 元気がない is a very common set phrase:

元気がある = to be lively / energetic / in good spirits
元気がない = to lack energy / not be lively / feel down / look unwell

Grammar point:

元気 here is a noun-like word that behaves a bit like a “subject” of existence.
が marks the thing that exists / doesn’t exist:
- 元気がある = there is (some) energy / spirit
- 元気がない = there is no energy / spirit

So:

元気をない is wrong; を is for direct objects, and ない here is not acting as a transitive verb.
In casual speech, people often drop が and say 元気ない in conversation, but the full grammatical form is 元気がない.

Nuance: 元気がない can mean physically tired, a bit sick, or emotionally down, depending on context. Here, from facial expression, it’s “she doesn’t look well / she looks down.”

Why is there a と before 分かる in 元気がないと分かる?

This と is the quotative particle. It often appears with verbs like:

言う (say)
思う (think)
分かる (understand / realize)
信じる (believe)

Structure:

X と分かる = understand / realize that X

So:

元気がないと分かる
= understand that (she) is not energetic
= you can tell she isn’t well / she doesn’t have much energy

Think of と here as marking what content is realized or understood.

Why is 分かる used instead of 知る?

Both can be translated “to know,” but they’re used differently.

知る is more like to learn / come to know a fact; it emphasizes acquiring information.
分かる is more like to understand / grasp / realize, often from context, evidence, or explanation.

In this sentence, you’re judging from her expression and “can tell / can see that…”:

元気がないと分かる
= you understand / realize that she isn’t energetic.

Using 知る here (元気がないと知る) would sound more like learning a factual piece of information, e.g., someone told you, rather than judging from appearance. So 分かる is the natural choice.

Is this sentence polite or casual? How would you make it more polite?

As written:

彼女の表情を見れば、一目で元気がないと分かる。

This is neutral/plain form. It’s suitable for narrative, explanation in writing, or casual speech.

To make it politely spoken (e.g., to a teacher or in formal context), you’d typically change the final verb to ます-form:

彼女の表情を見れば、一目で元気がないと分かります。
= If you look at her expression, you can tell at a glance that she isn’t well.

Everything else can stay the same; politeness is mainly carried by the verb at the end.

Can this sentence be rearranged or shortened in natural Japanese? For example, can parts be omitted?

Yes, Japanese allows some flexibility and omission, especially in conversation.

Examples:

Omit 彼女の if context is clear:
- 表情を見れば、一目で元気がないと分かる。
  If you look at (her) expression, you can tell at a glance she’s not well.
More casual, with って instead of と:
- 彼女の表情を見れば、一目で元気ないって分かる。
Drop 一目で if you don’t need “at a glance”:
- 彼女の表情を見れば、元気がないと分かる。

You generally keep the order:

[彼女の表情を見れば]、[一目で][元気がないと分かる]

Large reordering (like moving 一目で to the very front) is possible but not very natural here. The given order is standard and clear.

What is the overall literal structure of the sentence?

Breakdown:

彼女の表情を見れば、
- 彼女の表情: her facial expression
- を: object marker
- 見れば: if (one) looks at / when (one) looks at
一目で: by one glance / at a glance
元気がないと: (that) she has no energy / is not lively
分かる: (one) understands / can tell

Putting it together very literally:

If (you) look at her facial expression, at a glance, (you) understand that (she) has no energy.

Natural English:

If you look at her expression, you can tell at a glance that she isn’t feeling well.