watasi ha teisyoku wo tyuumonsuru tumori datta ndesu ga, kanozyo ga tabete ita karee ni simasita.

Because V-る + つもりだ means intend to do V.

So:

• 注文するつもりだった = I intended to order
• 注文したつもりだった would mean something closer to I thought I had ordered or I believed I had already ordered, which is a very different idea

With つもり, the verb form changes the meaning a lot:

• するつもり = intend to do
• したつもり = think that one did / act as if one did

So in this sentence, 注文するつもりだった is the natural choice because the speaker is talking about a plan they had before making a different choice.

つもりだった means was planning to or had intended to.

Breaking it down:

• つもり = intention, plan
• だった = past form of だ

So 注文するつもりだった means:

• I was going to order
• I had intended to order

The important nuance is that this was the speaker’s earlier intention, but it did not end up happening. The second half of the sentence explains what they actually chose instead.

んです adds an explanatory or background-giving tone.

So:

• 注文するつもりだったが sounds more direct and plain
• 注文するつもりだったんですが sounds softer and more like The thing is, I had been planning to order that, but...

In conversation, んです is very common when speakers:

• explain a situation
• give background
• lead into a contrast
• sound less abrupt

Here it helps connect the speaker’s original intention with the change of decision that follows.

Yes, here が works like but, but it is often softer than a direct English but.

In Japanese, が after an explanatory phrase often introduces:

• contrast
• hesitation
• a change of direction in the story

So だったんですが、... feels like:

• I had intended to order the set meal, but...
• I was going to order the set meal, though...

It can sound gentler and more natural in conversation than a very blunt contrast.

Because 彼女が食べていた is a clause modifying カレー, and inside that clause, 彼女 is the subject of 食べていた.

So:

• 彼女が食べていたカレー = the curry that she was eating

Inside relative clauses like this, が is very commonly used to mark the subject.

Using は is not impossible in some contexts, but here が is the most natural and neutral choice because it simply identifies she as the person doing the eating.

A useful way to see it is:

• [彼女が食べていた] カレー
• [she was eating] curry
• more naturally: the curry she was eating

食べていた is the past progressive / past ongoing form, so it means was eating.

• 彼女が食べていたカレー = the curry she was eating
• 彼女が食べたカレー = the curry she ate

In this sentence, 食べていた suggests the speaker saw her in the middle of eating it, and that influenced the speaker’s decision.

So the nuance is something like:

• I ended up choosing the curry she was eating

That feels more immediate and natural in this situation than 食べた, which would focus more on a completed action.

Noun + にする means to decide on, to choose, or to go with something.

So:

• カレーにしました = I chose curry / I went with the curry

This is a very common pattern when choosing food, drinks, options, colors, etc.:

• コーヒーにします = I’ll have coffee
• これにします = I’ll take this
• Mサイズにしました = I chose size M

So here に marks the thing the speaker settled on as their final choice.

Because する has different patterns depending on the meaning.

Here, する does not mean do in the ordinary sense. In Noun + にする, it means choose / make it X.

That pattern uses に, not を:

• 寿司にする = choose sushi
• 赤にする = choose red
• カレーにする = choose curry

If you used を, it would sound like do curry, which does not fit this meaning.

定食 is a set meal, usually a standard meal combination served together, often in restaurants.

It commonly includes things like:

• rice
• soup
• a main dish
• side dishes

So when the speaker says 定食を注文するつもりだった, they mean they had planned to order a set meal rather than curry.

Depending on context, it may not refer to one specific fixed item but rather the general category of set meal.

Yes, 私 could definitely be omitted if the subject is already clear from context.

Japanese often leaves out subjects when they are understood. So a very natural version in conversation could be:

• 定食を注文するつもりだったんですが、彼女が食べていたカレーにしました。

The sentence includes 私 probably for clarity, emphasis, or because it is being presented to a learner. In real conversation, it is often unnecessary.

This is completely normal in Japanese.

The final predicate of the sentence is polite:

• しました

And the explanatory part also uses polite style:

• んです

But forms inside modifiers or before certain grammar points are often plain:

• 注文する before つもり
• だった inside つもりだったんです
• 食べていた inside the clause modifying カレー

Japanese does not require every verb in the sentence to be in polite form. Usually, the overall politeness is shown mainly by the sentence ending.

So this mix is natural and grammatical.

It can mean either, depending on context.

• 彼女 = she
• 彼女 can also mean girlfriend

In this sentence, because it is followed by が食べていたカレー, it most naturally reads as she unless the wider context makes it clear that the woman is the speaker’s girlfriend.

So both are possible in theory, but she is the safer default if no extra context is given.

Literally, yes, it is built from that kind of structure, but the natural meaning is I chose the curry she was eating.

That is because Noun + にする often means decide on X rather than physically turn something into X.

So although the grammar can look strange if translated word-for-word, the idiomatic meaning is very common and natural in Japanese.

Not with the same meaning.

• が here shows contrast: I intended to order the set meal, but I chose the curry
• ので and から show reason: because

If you changed it to ので or から, the logic of the sentence would change. It would start sounding like the first part is the reason for the second part, which is not what the speaker means.

The speaker is contrasting:

• original plan: set meal
• final choice: curry

So が is the right connector here.