sakuya ha denki wo tukeppanasi ni site nete simaimasita.

昨夜は sets last night as the topic: As for last night, ...

Using は here helps frame the sentence as talking about what happened last night. It does not mean contrast is always strong, but it can lightly suggest speaking specifically about last night.

You would usually not use に with 昨夜 here. Time words like 今日, 昨日, 昨夜, 明日 often appear without に, especially when they refer to general time expressions rather than exact points in time.

Because 電気をつける means to turn on the light(s).

Here, 電気 is the thing being turned on, so it takes the object marker を.

A very common set is:

• 電気をつける = turn on the light(s)
• 電気を消す = turn off the light(s)

Even though 電気 literally means electricity, in everyday Japanese it often refers to the lights in context.

つけっぱなし means left on.

It comes from:

• つける = to turn on
• 〜っぱなし = leaving something as it is; doing something and not changing/fixing it afterward

So:

• つけっぱなし = leaving something turned on
• 開けっぱなし = leaving something open
• 出しっぱなし = leaving something out

This pattern often carries a nuance of carelessness, failure to deal with something, or leaving it that way unintentionally.

In つけっぱなしにする, the にする pattern means to make/leave something in that state.

So:

• つけっぱなし = in the state of being left on
• つけっぱなしにする = to leave it on

In this sentence, つけっぱなしにして is the て-form of つけっぱなしにする, linking it to the next action.

A helpful way to see it is:

• 電気をつけっぱなしにして = leaving the lights on

Japanese often links actions with the て-form.

Here the structure is:

• 電気をつけっぱなしにして
• 寝てしまいました

The first て-form connects the state/action leaving the lights on to the next action going to sleep.

So the flow is:

1. left the lights on
2. went to sleep
3. and unfortunately/regrettably ended up doing that

Japanese uses て-forms very naturally where English might use expressions like:

• and
• while
• after
• with
• in the state of

寝てしまいました adds a nuance that the action happened completely, and often that it was unfortunate, accidental, or regrettable.

Compare:

• 寝ました = I went to sleep
• 寝てしまいました = I ended up falling asleep / I unfortunately fell asleep

In this sentence, しまいました suggests something like:

• I fell asleep without meaning to
• I regret that I fell asleep with the lights still on
• Oops, I fell asleep

So しまう is doing important nuance work here.

Not always.

〜てしまう can mean different things depending on context:

1. Completion

   • 全部食べてしまいました = I ate it all up

2. Regret / unfortunate result

   • 財布をなくしてしまいました = I unfortunately lost my wallet

3. Accidental/unintended action

   • 寝てしまいました = I ended up falling asleep

In this sentence, the most natural feeling is regret/unintended result.

They are similar, but not exactly the same.

• つけたまま = left on / remaining on
• つけっぱなし = left on, often with a stronger feeling of carelessness or just leaving it like that

So both can work in similar situations, but っぱなし often sounds a bit more like:

• I left it on and didn’t deal with it
• I carelessly left it on

Compare:

• 電気をつけたまま寝た = I slept with the lights left on
• 電気をつけっぱなしにして寝た = I went to sleep leaving the lights on

The second often feels a little more blameworthy or negligent.

Because しまう attaches to the て-form of a verb.

So:

• 寝る
• 寝て
• 寝てしまう
• polite past: 寝てしまいました

This is a very common grammar pattern:

• 忘れてしまいました = I forgot
• 言ってしまった = I ended up saying it
• 食べてしまった = I ate it all / I went and ate it

So 寝てしまいました is just the normal way to combine 寝る with しまう.

The subject is omitted, which is very normal in Japanese.

From context, the subject is understood as I:

• I left the lights on and fell asleep.

Japanese often leaves out subjects when they are obvious from context. So even though there is no explicit 私 or 僕, the sentence is completely natural.

A useful breakdown is:

• 昨夜は = as for last night
• 電気を = the lights
• つけっぱなしにして = leaving them on
• 寝てしまいました = I ended up falling asleep

So the grammar is roughly:

[Time/topic] + [object] + [state left as-is] + [main action + しまう]

A natural literal gloss would be something like:

As for last night, I went to sleep having left the lights on, and unfortunately ended up falling asleep that way.

Yes. Casual versions include:

• 昨夜、電気をつけっぱなしにして寝ちゃった。
• 昨夜、電気つけっぱなしで寝ちゃった。

Here, 〜てしまった often becomes 〜ちゃった in casual speech:

• 寝てしまった → 寝ちゃった

That contraction is extremely common in everyday conversation.

Because しまいました is the polite past form of しまう.

So the sentence is polite because it ends with 〜ました. That is appropriate in:

• textbook sentences
• speaking politely
• talking to someone you are not very casual with

A plain-form version would be:

• 昨夜は電気をつけっぱなしにして寝てしまった。

Both mean basically the same thing; the difference is politeness level.

Yes. In English we often have to choose the light or the lights, but Japanese often leaves that unspecified.

So 電気をつけっぱなし could mean:

• left one light on
• left the room lights on
• left the electricity/lighting on

Context decides. In many translations, the lights sounds most natural.