tennai ga manseki datta node, motikaeri ni site saki ni kaikei wo simasita.

Here が marks what is being described as full.

• 店内 = the inside of the shop / restaurant
• 満席 = full, with no seats available

So 店内が満席だった literally feels like the inside was full or the restaurant was full.

You could also hear 店内は満席だった. The difference is subtle:

• が tends to present the situation as new information
• は would make 店内 the topic, sometimes with a slight contrastive feel

In this sentence, が is very natural because the speaker is explaining the situation that led to the next action.

満席 is a noun that means full house / no vacant seats. In Japanese, nouns like this can act like na-adjectives in predicates.

That is why you get:

• 満席だ = is full
• 満席だった = was full

So 店内が満席だった means the restaurant was full or more literally the inside had no open seats.

This word is especially common for places with seating, such as:

• restaurants
• trains
• theaters
• event venues

Both are possible.

• 満席だったので = plain-style past
• 満席でしたので = polite-style past

In Japanese, when ので connects clauses, the plain form before it is very common, even in otherwise fairly polite speech. So a sentence can mix:

• plain form before ので
• polite ending at the end, such as しました

That is exactly what is happening here:

• 満席だったので
• 会計をしました

This kind of mix is completely natural.

ので means because / since.

In this sentence, it gives the reason for the action that follows:

• 店内が満席だったので = because the restaurant was full

Compared with から, ので often sounds:

• a little softer
• more explanatory
• a little less blunt or personal

So:

• 満席だったので、持ち帰りにしました
  • softer, explanatory
• 満席だったから、持ち帰りにしました
  • also correct, but slightly more direct

Both are natural, but ので fits well when calmly explaining why something was done.

This comes from the pattern N にする, which means to choose N or to make it N.

So:

• 持ち帰りにする = to choose takeout / make it takeout

Then the て-form:

• 持ち帰りにして

connects that action to the next one:

• I chose takeout, and then...

So the structure is:

• 持ち帰りにして
• 先に会計をしました

meaning something like:

• I decided to get it to go and paid first

Good question. These are related, but they are not the same.

• 持ち帰る = to take something home / carry it away
• 持ち帰りにする = to choose the takeout option

In a restaurant or café context, 持ち帰りにする is the natural expression when deciding how the order will be handled.

So:

• 持ち帰りにします = I’ll have it to go
• 持ち帰ります can sound more like I will physically take it home

At the ordering stage, 持ち帰りにする is usually the more natural phrasing.

The に is part of the pattern N にする.

This pattern means:

• to make something into X
• to choose X
• to go with X

Examples:

• コーヒーにします = I’ll have coffee
• 大盛りにする = make it a large portion
• 持ち帰りにする = choose takeout

So に marks the result or choice. It is not a location marker here.

先に means first, ahead of time, or before that.

Here it means the speaker paid before whatever would normally come later. In context, that is likely:

• before receiving the food
• before leaving with it
• or before any other next step

So 先に会計をしました means:

• I paid first
• I took care of the payment beforehand

It shows order of actions very clearly.

Yes. 会計 is a noun meaning bill, checkout, or payment, and 会計をする means to pay / to settle the bill / to check out.

So:

• 会計をしました = I paid / I settled the bill

This is very common in restaurant or store situations.

A few related expressions:

• お会計 = bill / check / payment, often more polite
• 会計をお願いします = Check, please / I’d like to pay
• 払う = to pay, more general

So 会計をする is especially tied to the act of settling the bill at a register or restaurant.

Because Japanese often leaves out the subject when it is obvious from context.

In this sentence, the speaker is clearly talking about their own action:

• the restaurant was full
• so they chose takeout
• and they paid first

Because that is easy to infer, 私は is unnecessary.

If you added it, it would still be grammatical:

• 私は、店内が満席だったので、持ち帰りにして先に会計をしました。

But in natural Japanese, leaving it out is usually better unless you need emphasis or contrast.

It breaks down like this:

• 店内が満席だったので
  • because the restaurant was full
• 持ち帰りにして
  • I chose takeout, and...
• 先に会計をしました
  • I paid first

So the flow is:

1. reason
2. decision/action
3. next action

A very literal structure would be:

• Because the inside was full, making it takeout, I paid first

A more natural English version is:

• Because the restaurant was full, I decided to get it to go and paid first.

This is a very common Japanese pattern: reason clause + て-form connection + final main action.

Yes, 店内 literally means inside the store/shop. But in restaurant and café contexts, it often refers naturally to the dine-in space.

So when someone says:

• 店内が満席

they usually mean:

• the seats inside are all taken
• there is no room to eat inside

Japanese often uses a broader location word where English might say something more specifically like the dining area or the restaurant. In context, the meaning is very clear.