Learn Hardcore Japanese: kinou ha sinya made intaanetto de dougasaito wo mite simaimasita. - Yesterday I ended up watching video sites on the internet until late at night.

In 昨日は深夜までインターネットで動画サイトを見てしまいました, the subject is understood as “I” (or we, or someone specific) depending on the situation, but it’s not written.

If you really wanted to state it explicitly, you could say:

私は昨日は深夜までインターネットで動画サイトを見てしまいました。
“As for me, yesterday I (regrettably) watched video sites on the internet until late at night.”

But using 私 here is usually unnecessary and can sound a bit stiff or overly self-focused in many contexts. Native speakers normally omit it.

Why is it 昨日は and not 昨日に? I thought time expressions take に.

Both are possible, but they have different functions:

昨日に marks “yesterday” as a time point with the particle に:
- 昨日に帰りました。 – “I went home yesterday.”
  (Here, 昨日に is just “on yesterday / at yesterday”.)
昨日は uses は, which marks “yesterday” as the topic of the sentence:
- 昨日は深夜までインターネットで動画サイトを見てしまいました。
  “As for yesterday, I ended up watching video sites on the internet until late at night.”

Also, in practice:

Time expressions like 昨日, 今日, 明日, 毎日 often appear without any particle:
- 昨日深夜まで起きていました。 (Zero particle → just “yesterday” as a time adverb)

So your options are roughly:

昨日、深夜まで〜 (most natural, zero particle)
昨日は深夜まで〜 (topic “yesterday”)
昨日に深夜まで〜 (sounds awkward here; に is rarely used with 昨日 in this kind of sentence)

In this sentence, 昨日は emphasizes “about yesterday, this is what happened.”

What does 深夜 mean exactly? How is it different from just saying “night”?

深夜 (しんや) literally means “deep night”, i.e. very late at night.

Nuance comparison:

夜 (よる) – night in general (after evening, before bedtime).
夜中 (よなか) – middle of the night, late night, but quite casual and broad.
真夜中 (まよなか) – midnight / the dead of night (very strong “middle of the night” feeling).
深夜 (しんや) – late at night, often used for time slots like 深夜アニメ (late-night anime), 深夜番組 (late-night TV shows).

In this sentence, 深夜まで implies something like “until very late at night,” often with the nuance that it was maybe too late or not healthy/sensible.

What exactly does まで in 深夜まで mean?

まで means “until (a certain limit)”.

深夜まで = “until late at night / until the late-night hours”

So the action 見てしまいました (watched) continued up to that point.

Compare:

深夜まで起きていました。 – I stayed up until late at night.
深夜までにレポートを出してください。 – Please submit the report by late at night.

Here, までに = “by (before or at that time)”
while まで = “up to (all the way until that time).”

In this sentence, 深夜まで clearly means the watching continued up to that time, not just that it was finished by then.

Why is it インターネットで and not インターネットを? What does で do here?

The particle で has several uses; two relevant ones here are:

Place where an action happens
- 学校で勉強します。 – I study at school.
Means / medium / tool
- テレビで映画を見ます。 – I watch movies on TV.

In インターネットで動画サイトを見てしまいました, インターネットで combines both “place/medium”:

It can be read as “on the internet” (medium / environment where watching happens).

Using を after インターネット would make it the direct object (the thing you watch), which it isn’t. The thing you are directly watching is 動画サイト, so that rightly takes を:

インターネットで (on/through the internet – medium)
動画サイトを (video sites – direct object of 見る)

Isn’t saying インターネットで動画サイトを redundant? Aren’t video sites obviously on the internet?

In real life, yes, we normally assume 動画サイト are on the internet.
Grammatically, though, this structure is very natural:

インターネットで動画サイトを見ました。
literally: “I watched video sites on the internet.”

You can think of it like English:

“I watched videos on streaming sites on the internet.”
“I browsed shopping sites on the internet.”

インターネットで tells you how/where the action is done (online, via the internet), and 動画サイトを tells you what you watched.

You could also drop インターネットで and just say:

昨日は深夜まで動画サイトを見てしまいました。

This would usually still be understood as “online video sites” from context, but including インターネットで emphasizes the “I was on the internet” aspect.

What does 動画サイト mean? Is it singular or plural?

動画サイト literally means “video site” (a website for streaming/hosting videos), e.g. YouTube, Nico Nico Douga, etc.

Japanese nouns do not mark singular/plural by default, so:

動画サイト can mean “a video site”, “the video site”, or “video sites”.
The context (and sometimes words like いろいろな, 何個も, たくさん) decides the exact nuance.

In this sentence, 動画サイトを見てしまいました is naturally understood as:

“I was watching (stuff on) video sites” (plural sense), or
“I watched a video site (for a long time)” (less common reading, but possible).

English almost always chooses plural in this context: “video sites” or just “videos (on video sites).”

What nuance does 〜てしまいました add compared to just 見ました?

〜てしまう (in 〜てしまいました) as an auxiliary verb carries two main nuances:

Completion: doing something completely / entirely
Regret / unintendedness: doing something by accident, more than intended, or with some negative feeling about it

In this sentence:

見ました。 – I watched (plain past, neutral).
見てしまいました。 –
- “I ended up watching,”
- “I watched (too long / when I shouldn’t have),”
- “I (unfortunately) watched.”

Given 深夜まで (“until very late at night”), the sentence strongly suggests regret / “I overdid it”:

“Yesterday, I ended up watching video sites on the internet until late at night (and I kind of regret it / it wasn’t a good idea).”

If the speaker had no negative feeling, 見ました would be more neutral.
見てしまいました adds that sense of “oops / I shouldn’t really have done that” or “I got carried away.”

Why is the verb in the て-form (見て) before しまいました?

〜てしまう is formed by:

Taking the verb’s て-form, e.g.
- 見る → 見て
- 食べる → 食べて
- 飲む → 飲んで
Adding しまう (here in polite past: しまいました)

So:

見てしまう – to end up watching, to (unfortunately) watch
見てしまいました – past polite: “ended up watching,” “(regrettably) watched”

This is a very common pattern in Japanese:

全部食べてしまいました。 – I ended up eating it all / I (regrettably) ate it all.
財布をなくしてしまいました。 – I (unfortunately) lost my wallet.

The て-form here is not just “and”; it’s how you attach the auxiliary しまう to the main verb.

Can I change the word order of this sentence?

Japanese word order is fairly flexible as long as:

The verb comes at the end, and
The particles stay attached to the right words.

Your original sentence:

昨日は深夜までインターネットで動画サイトを見てしまいました。

Some possible variations:

昨日はインターネットで深夜まで動画サイトを見てしまいました。
昨日は深夜まで動画サイトをインターネットで見てしまいました。 (less natural, but grammatical)
深夜まで昨日はインターネットで動画サイトを見てしまいました。 (odd in normal speech, but possible with special emphasis)

The most natural flows keep time expressions toward the front and keep related phrases together:

[Time] 昨日は
[Until when] 深夜まで
[Medium] インターネットで
[Object] 動画サイトを
[Verb] 見てしまいました

So the original order is already very natural and clear.

What level of politeness is 見てしまいました? How would it sound in casual speech?

見てしまいました is:

Polite past (〜ます form)
Suitable for talking to people you are not very close to, or in any neutral/formal situation.

Casual equivalents:

見てしまった。 – casual past; same nuance of completion/regret.
見ちゃった。 – contraction of 見てしまった; very casual, spoken, often used among friends/family.

昨日は深夜までインターネットで動画サイトを見てしまいました。
→ polite, neutral/formal.
昨日は深夜までインターネットで動画サイト見てしまった。
→ casual, plain.
昨日、深夜までネットで動画サイト見ちゃった。
→ very casual, conversational, sounds like talking to a friend.

The meaning is basically the same; only the politeness and style change.