tennai ga manseki datta node, sandoitti wo motikaeri ni simasita.

店内 is read てんない.

It means inside the store/shop. In this sentence, it specifically refers to the eat-in area inside the shop. So 店内が満席だった means that the seating inside the shop was full.

Here, が marks 店内 as the thing being described by 満席だった.

The pattern is basically:

A が 満席だ = A is full / has no seats available

So:

店内が満席だった = the inside seating was full

If you used は instead, it would sound more like you are setting up 店内 as the topic, possibly with some contrast:

• 店内は満席だった = As for the inside seating, it was full

Both are possible, but が works naturally when presenting the situation as the reason for what happened next.

満席 means full house, no seats available, or all seats occupied.

It is not an -i adjective. It behaves like a noun / na-adjective-type word, so when it is used as the predicate, it takes the copula:

• 満席だ
• 満席です
• 満席だった
• 満席でした

So 満席だった is simply the past tense form: was full.

Yes, that is completely normal.

In Japanese, clauses before connectors like ので, から, けど, が, etc. are very often in the plain form, even when the main sentence ends politely.

So this is natural:

• 満席だったので、持ち帰りにしました。

The first part is a subordinate clause, so だった is expected. The main sentence ends with the polite しました.

This kind of mix is standard Japanese, not a mistake.

Both ので and から can mean because, but the nuance is different.

• ので sounds a little more explanatory, softer, and more objective
• から can sound a little more direct, personal, or assertive

So:

• 店内が満席だったので、サンドイッチを持ち帰りにしました。

has a calm, matter-of-fact tone: Since the shop was full, I decided to get the sandwich to go.

Using から would also be grammatical, but ので sounds a bit gentler and more natural in this kind of explanation.

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

In this sentence, the understood subject is something like I or we, depending on the situation:

店内が満席だったので、サンドイッチを持ち帰りにしました。 = Because the shop was full, I/we decided to get the sandwich to go.

Japanese usually does not repeat subjects unless needed for clarity or contrast.

Here, 持ち帰り is a noun, not a verb.

It means takeout, to-go, or taking something home.

It comes from the verb 持ち帰る, which means to take something home. But in this sentence, 持ち帰り is being used as a noun in the expression 持ち帰りにする.

So it is similar to saying:

• make it takeout
• have it to go

This uses the pattern:

A を B にする

which means to make A into B or to choose B for A.

So:

• サンドイッチを持ち帰りにしました

literally has the structure:

• made the sandwich into takeout
• more naturally: decided to get the sandwich to go

The に marks the result or chosen form: takeout.

You can think of it as similar to:

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

Because サンドイッチ is the thing being affected by the choice.

In A を B にする, を marks the item, and B にする tells you what you make it or choose it as.

So:

• サンドイッチを持ち帰りにしました

means that the sandwich is the item, and takeout is the chosen form.

In other words: I chose the sandwich as takeout.

Yes, but it means something slightly different.

• 持ち帰りにしました = I decided to make it takeout / I got it to go
• 持ち帰りました = I took it home

So the sentence in your example focuses on the decision/order choice, not the physical action afterward.

Compare:

• 店内が満席だったので、サンドイッチを持ち帰りにしました。
  Because the shop was full, I got the sandwich to go.

• サンドイッチを持ち帰りました。
  I took the sandwich home.

The first is about choosing takeout. The second is about actually carrying it away.

Because the speaker is talking about what happened in a past situation:

• the shop was full
• so they chose takeout

That is why you get:

• 満席だった
• しました

If you were talking about the present situation, you might say:

• 店内が満席なので、サンドイッチを持ち帰りにします。

That would mean something like: Since the shop is full, I’ll get the sandwich to go.

Yes, and it is natural.

Japanese often uses slightly compressed expressions like this. Strictly speaking, what is full is really the seats inside the shop, but Japanese does not need to spell that out every time.

So 店内が満席 naturally means:

• the eat-in area was full
• there were no seats available inside

A more explicit version might be something like 店内の席が満席, but that is usually unnecessary. The shorter version sounds natural and normal.