kinou ha tukarete ita node, sofa de nete simaimasita.

は is marking 昨日 as the topic: as for yesterday.

A time word like 昨日 can often appear with no particle at all, so 昨日疲れていたので... is also possible. But adding は does a few useful things:

• it sets the time frame clearly
• it gives a slight contrastive feeling, like yesterday, at least...
• it makes the sentence sound a bit more organized as a topic-comment structure

So 昨日は is very natural here.

疲れていた comes from the verb 疲れる, which means to get tired.

The pattern is:

• 疲れる = get tired
• 疲れている = be tired
• 疲れていた = was tired

This can feel strange to English speakers, because Japanese often uses 〜ている not only for ongoing actions, but also for resulting states.

So here, 疲れていた does not mean was getting tired. It means was in the state of being tired.

Both are possible in some situations, but the nuance is different.

• 疲れた = got tired / became tired / I'm tired
• 疲れていた = was tired

In this sentence, the speaker is giving background information that explains what happened next. Because of that, 疲れていた sounds very natural: it describes the ongoing condition that led to falling asleep on the sofa.

So:

• 疲れたので = because I got tired / because I was tired
• 疲れていたので = because I was tired

The second one sounds more like a state that was already true.

ので means because or since.

It connects a reason to a result:

• 疲れていたので = because I was tired
• ソファで寝てしまいました = I ended up falling asleep on the sofa

Compared with から, ので often sounds:

• a little softer
• more explanatory
• less blunt

So it is very common when giving a reason in a natural, polite way.

Yes, you could say:

昨日は疲れていたから、ソファで寝てしまいました。

This is grammatical and natural. The difference is mostly nuance:

• から = more direct, more straightforward
• ので = softer, more explanatory

So in this sentence:

• ので sounds a bit gentler
• から sounds a bit more conversational and direct

Neither is wrong.

This is a very common pattern in Japanese.

In a sentence like this, the clause before ので usually uses the plain form, while the main clause can still be polite.

So:

• 疲れていた = plain past
• 寝てしまいました = polite past

This is completely normal Japanese.

You do not need to make everything polite all the way through. In fact, something like 疲れていましたので、寝てしまいました is possible, but it sounds more formal or stiff in everyday conversation.

So the sentence is natural because:

• the subordinate clause uses plain form
• the final verb carries the politeness

で marks the place where an action happens.

Here, the action is 寝る: sleeping. The sofa is the location of that action, so ソファで is natural.

• ソファで寝る = sleep on the sofa

Using に here is usually unnatural with 寝る in this meaning.

As a general idea:

• で = where an action takes place
• に = destination, target point, existence, or other specific roles

So for this sentence, ソファで寝る is the standard choice.

〜てしまう adds extra nuance.

• 寝ました = slept
• 寝てしまいました = ended up sleeping / fell asleep

In many cases, 〜てしまう suggests one of these ideas:

• something finished completely
• something happened unintentionally
• the speaker feels slight regret, surprise, or inconvenience

Here, it strongly suggests:

• the speaker did not really plan to sleep there
• they ended up falling asleep on the sofa

That is why it sounds so natural in this sentence.

Often, yes, at least a little.

〜てしまう does not always mean strong regret, but it very often suggests:

• oops
• I didn't mean for that to happen
• it happened before I realized it

So in this sentence, the feeling is something like:

• I was tired, so I ended up falling asleep on the sofa

That can imply mild regret, embarrassment, or just unintended completion.

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

From context, the understood subject is most likely:

• I
• or the speaker

So English needs to say I was tired, but Japanese often leaves that unsaid when it is obvious.

This is one of the biggest differences from English. Japanese frequently omits:

• subjects
• objects
• pronouns

if the listener can understand them from context.

No, it is not strictly necessary, but it is very common.

The comma helps separate:

• the reason part: 昨日は疲れていたので
• the main result: ソファで寝てしまいました

It makes the sentence easier to read and gives a natural pause.

So both are possible:

• 昨日は疲れていたので、ソファで寝てしまいました。
• 昨日は疲れていたのでソファで寝てしまいました。

The version with the comma is often easier to process.

Japanese usually puts the background information before the main action, and the main verb comes at the end.

So the structure is:

• 昨日は = as for yesterday
• 疲れていたので = because I was tired
• ソファで = on the sofa
• 寝てしまいました = ended up falling asleep

A natural way to think about it is:

1. set the time
2. give the reason
3. give the location
4. finish with the main action

That is very typical Japanese sentence structure.