tennai ga manseki datta node, motikaeri ni simasita.

In this sentence, 店内が満席だった means the inside seating was full.

The particle が marks what is in the state of being full. So the structure is basically:

• 店内 = the inside of the shop / restaurant
• が
• 満席 = full, no seats available
• だった = was

A very natural English way to understand it is As for the dine-in area, it was full or The restaurant was full inside.

You could also see は in similar sentences, but が is natural when presenting the situation as the reason for what follows.

店内 literally means inside the shop/store.

In this context, it refers to the inside seating area of a restaurant or café, so it is often understood as dining in or the eat-in area.

That is why the sentence contrasts naturally with 持ち帰り:

• 店内 = eat inside
• 持ち帰り = takeout / takeaway

So even though 店内 literally means inside the store, in food-related situations it often implies eating there.

満席 is a very common word meaning all seats are occupied or full house.

It is specifically about seats, so it is perfect for restaurants, cafés, theaters, trains, and so on.

Examples:

• 店内が満席です = The restaurant is full
• 電車は満席です = The train is fully booked / all seats are taken

It is more specific than a general word like いっぱい, because 満席 clearly tells you that the problem is there are no seats available.

満席だった is the plain form, while 満席でした is the polite form.

Here, だった appears before ので, and it is very common to use the plain form in this position, even when the main clause is polite:

• 店内が満席だったので、持ち帰りにしました。

This mix is completely natural:

• subordinate clause: plain form
• main clause: polite form

If you wanted the whole sentence to sound more formal, you could also say:

• 店内が満席でしたので、持ち帰りにしました。

That is also correct, but the original sentence is very natural everyday Japanese.

ので means because or since.

So:

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

Compared with から, ので usually sounds:

• a little softer
• a little more explanatory
• a little less direct

Compare:

• 満席だったので、持ち帰りにしました。
  Because it was full, I chose takeout.

• 満席だったから、持ち帰りにしました。
  Because it was full, I chose takeout.

Both are correct, but ので often sounds a bit more polite or neutral.

にする often means to decide on, to choose, or to go with.

So:

• 持ち帰りにしました = I chose takeout / I decided on takeout

This is a very common pattern:

• A にする = choose A
• コーヒーにします = I’ll have coffee
• これにします = I’ll take this one
• 持ち帰りにしました = I decided on takeout

Literally, it is something like I made it takeout, but in natural English that becomes I chose takeout.

Because this is not the verb to do takeout. It is the pattern Noun + にする, meaning choose/make it noun.

So:

• 持ち帰りにしました = I made it takeout / I chose takeout

If you say 持ち帰りをしました, it sounds like you are treating 持ち帰り as something you directly did, which is not the natural expression here.

In restaurant situations, the standard phrasing is:

• 店内でお願いします = For here, please
• 持ち帰りでお願いします = To go, please
• 持ち帰りにします = I’ll make it takeout / I’ll choose takeout

Japanese often leaves out information that is obvious from context.

In 持ち帰りにしました, the speaker does not explicitly say what they made takeout. It could be understood as:

• the order
• the meal
• the purchase
• the plan

In English we often need to say more, but in Japanese it is very natural to leave it out if the situation is clear.

So the sentence might be understood more fully as something like:

• 店内が満席だったので、注文は持ち帰りにしました。
• Because the restaurant was full, I made my order takeout.

But the shorter original version is much more natural.

Yes. Japanese very often omits the subject when it is obvious.

In 持ち帰りにしました, the person who made the decision is not stated, but from context it is usually understood as:

• I
• we
• or whoever is speaking

So depending on the situation, the English translation could be:

• I decided to get takeout
• We decided on takeout

Japanese does not need to say 私は unless there is a reason to emphasize it or contrast it with someone else.

Yes, 店内は満席だったので is possible.

The difference is subtle:

• 店内が満席だったので focuses on the fact that the inside seating was full
• 店内は満席だったので can sound more like as for the inside, it was full, possibly with a contrast in mind

For example, は might feel a little more natural if you are contrasting 店内 with something else, such as outside seating or takeout.

But in the original sentence, が is perfectly natural and very common.

It is a mix, but a very normal mix.

• 満席だったので uses plain style in the reason clause
• しました uses polite style in the main clause

This is common Japanese. The main clause often carries the politeness level of the sentence, while earlier dependent parts may stay in plain form.

So the sentence sounds polite overall because it ends with しました.

If you wanted it more casual:

• 店内が満席だったので、持ち帰りにした。

If you wanted it more formal:

• 店内が満席でしたので、持ち帰りにしました。

Yes. Both are common.

• 持ち帰り = native Japanese expression, very common
• テイクアウト = loanword from English, also very common

So these are both natural:

• 持ち帰りにしました。
• テイクアウトにしました。

Depending on the shop, one may sound more natural than the other, but both are widely understood. 持ち帰り can sometimes feel a little more traditional or straightforward, while テイクアウト is common in modern restaurant language.