kono pasuta wo tyuumonsitaindesu ga, mada arimasu ka?

を marks the direct object of the verb. In this sentence, the thing being ordered is このパスタ, so を shows that it is the item affected by 注文したい.

• このパスタを注文したい = I want to order this pasta
• Here, 注文する means to order, and the thing you order takes を

So このパスタを is the natural pattern.

Both are possible, but 注文したいんです sounds a little softer and more explanatory.

• 注文したいです = I want to order it
• 注文したいんです = I want to order it / The thing is, I'd like to order it

The んです part often adds a feeling of giving background, explanation, or setting up what comes next. Since the speaker is about to ask まだありますか, it sounds very natural to use んです here.

んです adds an explanatory or contextual tone. It often feels like:

• the thing is...
• you see...
• actually...

In this sentence, the speaker is not just making a flat statement. They are saying something like:

• The thing is, I'd like to order this pasta, but... do you still have it?

This makes the sentence sound more natural in conversation, especially when leading into a question or request.

Yes, が can mean but, but in sentences like this it often works more softly than English but.

Here it connects:

• このパスタを注文したいんですが
• まだありますか

So the full feeling is:

• I'd like to order this pasta, but do you still have it?

In Japanese, が is often used to soften what follows or to introduce a question politely. It can sound less direct than simply asking the question immediately.

That is basically correct. Literally, まだありますか is something like:

• Is it still there?
• Does it still exist?

But in a restaurant context, that naturally means:

• Do you still have it?
• Is it still available?

So あります is not only about physical existence. It can also mean that something is available or in stock, depending on context.

Japanese often omits words that are clear from context. After saying このパスタを注文したい, it is obvious that まだありますか is asking about that pasta.

English often repeats the object:

• Do you still have it?

Japanese often leaves it out if everyone understands what it is.

If you wanted to say it more explicitly, you could say something like:

• このパスタはまだありますか
• このパスタはまだありますか

But in conversation, omission is very common and natural.

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

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

• 注文したい = want to order

So there is no need to say 私は. Adding it is possible, but it often sounds unnecessary unless you want contrast or emphasis.

• 私はこのパスタを注文したいんですが...
  This is grammatical, but usually less natural in a simple restaurant situation.

このパスタ is understandable and natural enough, especially if you are pointing at a menu item. But in a restaurant, people often use slightly more polite expressions such as:

• こちらのパスタ = this pasta here
• このパスタ = this pasta

So a more polite version might be:

• こちらのパスタを注文したいんですが、まだありますか。

That said, このパスタ is not wrong.

No, 注文したいです / 注文したいんです is generally fine and understandable. It means I’d like to order.

However, Japanese often prefers slightly more indirect or polished expressions in service situations. For example:

• このパスタをお願いします = This pasta, please
• このパスタを注文してもいいですか = May I order this pasta?
• こちらのパスタをお願いしたいんですが = I’d like to ask for this pasta

So 注文したいんです is not rude, but there are other expressions that may sound smoother or more service-oriented.

Yes. In context, it sounds like you are asking whether the item is still available.

This is very natural if:

• the restaurant may have run out of it
• it is a limited item
• it is late in the day
• you are unsure whether it is still being served

If you want a clearer expression specifically about availability, you could also hear:

• まだありますか = Is it still available?
• まだ注文できますか = Can I still order it?
• もう売り切れですか = Is it already sold out?

Japanese normally puts the verb near the end of the clause. That is one of the biggest word-order differences from English.

English:

• I want to order this pasta

Japanese:

• このパスタを注文したい

A very rough breakdown is:

• このパスタ = this pasta
• を = object marker
• 注文したい = want to order

Then the next clause also ends with the verb:

• まだありますか = is it still available?

So Japanese often follows an object + verb pattern where English uses verb + object.

ございますか is a more polite version of ありますか.

So:

• まだありますか = polite standard
• まだございますか = more formal / more polite

In a restaurant, both can work, but ありますか is very common and completely acceptable for learners. If you want to sound more polished, you could say:

• こちらのパスタを注文したいんですが、まだございますか。

That sounds more formal and customer-service-like.