Learn Hardcore Japanese: sinya ni dougasaito wo miru to, me ga tukareru desyou. - If you watch video sites late at night, your eyes will probably get tired.

深夜に動画サイトを見る = to watch video sites *at late night*
The pattern is the same as:
- 七時に起きる – wake up at 7
- 週末に映画を見る – watch a movie on the weekend

Omitting に:

In casual speech, 深夜動画サイトを見ると… is possible and often heard.
However, with more complex sentences or when you want to be clear and natural, keeping に after a time word is safer and more standard.

So 深夜に is the normal way to say “late at night / at midnight” as a time expression.

What is the function of と after 見る? Does it mean “if” or “when”?

Here 見ると is the conditional と.

V-辞書形 (dictionary form) + と often means:
- whenever / when / if (as a rule, whenever X happens, Y happens)

So:

深夜に動画サイトを見ると、目が疲れるでしょう。
≈ When/whenever you watch video sites late at night, your eyes will probably get tired.

Key nuance:

This と is used for natural, automatic, or typical results:
- 春になると、暖かくなる。 – When it becomes spring, it gets warm.
- ボタンを押すと、ドアが開く。 – If/when you press the button, the door opens.

That fits the idea that “if you do this (watch late at night), that naturally happens (eyes get tired).”

Why are 見る and 疲れる in plain (dictionary) form instead of 見ます and 疲れます?

Two things are going on:

Conditional と uses the plain form before と
- After a verb, と takes the plain form: 見ると, not 見ますと (except in special polite/business styles).
- So 見ると is the standard conditional form.
でしょう attaches to the plain form in ordinary polite speech
- You usually say:
  - 疲れるでしょう (natural)
  - 疲れますでしょう (possible but sounds very formal / stiff, often in customer service, not normal conversation)

So the pattern is:

[plain form] と、[plain form] でしょう。
- 深夜に動画サイトを見ると、目が疲れるでしょう。

The sentence is still overall polite because of でしょう, even though the verbs themselves are in dictionary form.

Why is 動画サイト marked with を and not で? What’s the difference?

を marks the direct object of the verb.

動画サイトを見る = to look at / watch a video site
(動画サイト is what you are watching.)

で marks the place or means where an action happens.

動画サイトで映画を見る = to watch a movie *on a video site
(*動画サイト is the platform/place where you watch something else.)

In this sentence:

動画サイトを見る
= you’re simply “watching video sites” themselves (browsing YouTube, NicoNico, etc.).

If you said 動画サイトで見る, you’d expect something after it, like:

深夜に動画サイトで映画を見ると… – If you watch movies on video sites late at night…

So を is correct because 動画サイト is the thing being watched.

Why does the sentence use 目が疲れる and not 目は疲れる?

が here marks what becomes tired as the subject/focus of the state.

目が疲れる = your eyes get tired (eyes are the part that experience tiredness)

If you use は, it usually marks a topic or a contrast:

目は疲れる。
= As for my eyes, they get tired (but maybe something else doesn’t / I want to contrast this with something).

In this context, the speaker is just describing the result “what gets tired”: the eyes. There’s no special contrast, so 目が疲れる is the natural choice.

What nuance does でしょう add? Is it like “probably”, or like a question tag (“right?”)?

でしょう has two main uses, both depending on context and intonation:

Guess / probability (neutral tone)
- Roughly “probably / I think / I suppose”
- 目が疲れるでしょう。
  = Your eyes will probably get tired / Your eyes tend to get tired.
Seeking confirmation / softening, like “right?” (rising intonation in speech)
- 目が疲れるでしょう？
  = Your eyes get tired, right?

In a written, neutral sentence like this, it’s mostly:

a soft, polite prediction, not a hard statement.
Softer than 目が疲れます (which sounds like stating a fact).

There is no “your/my” in the Japanese sentence. Whose eyes are 目?

Japanese often omits possessive pronouns like “my”, “your”, “his”, “her” when they are clear from context.

目が疲れるでしょう。
Literally: eyes will get tired, probably.

Who do they belong to?

Usually interpreted as the listener’s eyes (or “people’s eyes in general”), because the condition is something you do: when you watch video sites late at night…
If needed, Japanese can specify:
- あなたの目が疲れるでしょう。 – Your eyes will probably get tired.
- But adding あなたの often sounds too direct or even slightly rude in casual conversation, so it’s usually left out.

Context (who is speaking to whom, about what) usually tells you whose “eyes” they mean.

Could we use たら or なら instead of と? How would that change the nuance?

Yes, you can, but the nuance changes slightly.

と (in the original)
- Emphasizes a regular or automatic result:
  - Whenever you do X, Y happens (as a rule).
たら
- 深夜に動画サイトを見たら、目が疲れるでしょう。
- More like “if/when you (happen to) watch video sites late at night, your eyes will probably get tired.”
- Often feels a bit more one-time / event-based than a general rule.
なら
- 深夜に動画サイトを見るなら、目が疲れるでしょう。
- “If it’s the case that you watch video sites late at night, (then) your eyes will probably get tired.”
- Feels like you’re taking up a given condition or suggestion and commenting on it.
  (e.g., “If you’re going to do that, then here is what will happen.”)

The original と fits well because eye strain here is presented as a typical, natural consequence.

Is the word order fixed? For example, can we say 動画サイトを深夜に見ると instead?

Word order in Japanese is relatively flexible as long as the particles stay the same.

These are all grammatical and close in meaning:

深夜に動画サイトを見ると、目が疲れるでしょう。
動画サイトを深夜に見ると、目が疲れるでしょう。

Subtle differences:

Putting 深夜に first (深夜に動画サイトを…) slightly emphasizes “late at night” as the setting.
Putting 動画サイトを first (動画サイトを深夜に…) can slightly highlight what you’re watching first, then when.

In everyday conversation, both orders sound natural. The most important thing is using the correct particles (に, を, が).

What exactly does 深夜 mean, compared to 夜 or 夜中?

All three refer to “night”, but with different nuances:

夜 (よる)
- General “night / evening”.
- From evening to before very late at night; quite broad.
深夜 (しんや)
- Literally “deep night”.
- Usually implies very late at night, often after 11 p.m. or midnight.
- Used in schedules: 深夜番組 (late-night TV program), 深夜バス (late-night bus).
夜中 (よなか)
- “Middle of the night”.
- Also means late at night, often when most people are asleep.

In this sentence, 深夜 emphasizes that it’s quite late, not just any time in the evening—making it more believable that your eyes will get tired.