kono heya ha hontou ni sizuka de, yoru mo yoku nemureru.

Because 本当 is being used adverbially in 本当に, meaning really or truly.

• 本当 by itself often means truth or real / true
• 本当に means really

So in 本当に静か, it means really quiet.

This is a very common pattern:

• 静か = quiet
• 本当に静か = really quiet

静か is a na-adjective, not an i-adjective.

That means:

• dictionary form: 静か
• before a noun: 静かな部屋
• connective form: 静かで

So 静かで is the correct way to link quiet to what comes next.

Compare:

• 部屋は静かで、広い。 = The room is quiet and spacious.
• 部屋は静かで、夜もよく眠れる。 = The room is quiet, and I can sleep well at night too.

If it were an i-adjective, then 〜くて would be used instead:

• 寒くて眠れない = It’s cold, so I can’t sleep.

Here, で connects the first idea to the second idea.

So:

• 本当に静かで = being really quiet / and it’s really quiet
• 夜もよく眠れる = I can sleep well at night too

It can sound like:

• It’s really quiet, so...
• It’s really quiet, and...

In many sentences like this, the connection suggests a reason, even if Japanese does not explicitly say because.

は marks この部屋 as the topic of the sentence.

So the sentence is talking about this room and then making comments about it:

• この部屋は...静かで...眠れる
• As for this room, it’s really quiet, and...

This is very natural in Japanese. The topic is introduced first, and then the speaker says things about it.

も means also, too, or sometimes adds emphasis.

So 夜もよく眠れる suggests something like:

• I can sleep well at night too
• Even at night, I can sleep well

The nuance depends on context.

Common interpretations:

1. also/too: maybe the room is comfortable in general, and at night too it helps you sleep well
2. even: night can be a time when noise is a problem, but even at night you can sleep well

Without context, も adds a sense that night is included as well, not just stated neutrally.

眠れる is the potential form of 眠る, meaning can sleep.

So:

• 眠る = to sleep
• 眠れる = can sleep

That is why the sentence expresses ability or possibility:

• 夜もよく眠れる = can sleep well at night

This sounds more natural here than just 夜もよく眠る, which would mean something more like I sleep well at night as a habitual fact. 眠れる often suggests that the conditions allow it.

Yes. Japanese often leaves the subject unstated when it is obvious from context.

In English, we usually need to say:

• I can sleep well at night
• You can sleep well at night
• One can sleep well at night

In Japanese, the sentence just says よく眠れる, and the listener understands the subject from the situation. In this kind of sentence, it often means I can sleep well.

Here, よく means well.

So:

• よく眠れる = can sleep well

Be careful: よく can mean different things depending on context:

• often: よく行く = go often
• well: よく見える = can see well
• carefully / properly: よく考える = think carefully

In this sentence, it clearly means well.

It can be understood as both, but most naturally it feels like the first supports the second.

So the nuance is:

• This room is really quiet, so I can sleep well at night.

Japanese often uses the て-form / で-form to connect ideas without explicitly stating the exact relationship. The listener infers whether it means:

• and
• so
• because
• while

Here, quietness naturally explains being able to sleep well, so a reason-like reading is most natural.

This sentence is in a plain style rather than a polite です / ます style.

A polite version could be:

• この部屋は本当に静かで、夜もよく眠れます。

Both are correct. The difference is tone:

• 眠れる = plain / casual
• 眠れます = polite

Japanese often mixes plain description with context-dependent politeness, but as a learner, it is easiest to think of this sentence as a plain-style sentence.

Yes, but the nuance may shift slightly.

In this sentence, 本当に most naturally modifies 静か:

• この部屋は本当に静かで... = This room is really quiet...

If you moved it, it could sound like it modifies a different part:

• 夜も本当によく眠れる = I really can sleep well at night

So the current placement clearly emphasizes the room’s quietness.

この means this, so この部屋 means this room.

It points to a specific room that is close to the speaker, physically or in the conversation.

Compare:

• この部屋 = this room
• その部屋 = that room
• あの部屋 = that room over there

Using この makes the sentence more specific: the speaker is talking about one particular room, not rooms in general.