Wǒ búdàn xiǎng qù chāoshì, érqiě yào mǎshàng mǎi shuǐ.

不但……而且…… is a very common pattern meaning not only..., but also...

In this sentence:

• 我不但想去超市 = I not only want to go to the supermarket
• 而且要马上买水 = but also need/want to buy water right away

So the structure is:

• 不但 + point 1
• 而且 + point 2

It connects two related ideas, with the second one usually adding something stronger, more important, or more surprising.

This is because of tone sandhi, a pronunciation change.

Normally:

• 不 = bù (4th tone)

But when 不 comes before another 4th-tone syllable, it changes to 2nd tone:

• 但 = dàn (4th tone)
• so 不但 is pronounced búdàn

This happens in many common words and phrases, for example:

• 不是 → bú shì
• 不要 → bú yào
• 不对 → bú duì

So the character is still 不, but the pronunciation changes.

They can both relate to wanting, but they are not exactly the same.

In this sentence:

• 想去超市 = want to go to the supermarket
• 要马上买水 = need to / want to / am going to buy water right away

A useful way to feel the difference is:

• 想 often expresses a wish, idea, or desire
• 要 is often stronger and can sound more like need to, intend to, or be going to

So here:

• 想去超市 sounds like the speaker has the desire or plan to go
• 要马上买水 sounds more urgent and definite: buying water is something that must happen right away

That urgency is reinforced by 马上.

马上 means immediately, right away, or at once.

In this sentence:

• 要马上买水 = need to buy water right away

It usually appears before the verb phrase it modifies. Here it comes before 买.

A simple pattern is:

• 要 + 马上 + verb

Examples:

• 我要马上走。 = I need to leave right away.
• 他马上回来。 = He’ll come back immediately.

So in your sentence, 马上 tells us the action 买水 is urgent.

In Chinese, if the subject stays the same, it is often omitted the second time.

So:

• 我不但想去超市，而且要马上买水。

is completely natural, because both actions are done by 我.

You could also say:

• 我不但想去超市，而且我还要马上买水。

This is grammatical too, but it is a bit more explicit and slightly heavier. In everyday speech, Chinese often avoids repeating the subject unless it helps clarity or emphasis.

Many learners notice patterns like:

• 不但……而且还……

Yes, 还 is very common here, but it is optional in many sentences.

So all of these can be natural:

• 我不但想去超市，而且要马上买水。
• 我不但想去超市，而且还要马上买水。

Adding 还 can make the “in addition / furthermore” feeling a little stronger.

Very roughly:

• 而且 = moreover / and also
• 而且还 = and what’s more / and also additionally

But leaving out 还 is perfectly fine.

Chinese does not use articles like a, an, and the the way English does.

So 去超市 simply means go to the supermarket or go to a supermarket, depending on context.

Chinese often leaves this kind of definiteness unstated unless it matters.

If you wanted to be more specific, you could say things like:

• 去那个超市 = go to that supermarket
• 去一家超市 = go to a supermarket

But in your sentence, 去超市 is perfectly natural and general.

Measure words are usually needed with numbers or demonstratives, not necessarily with a bare noun as the object of a verb.

So:

• 买水 = buy water

is completely normal.

You would use a measure word in phrases like:

• 买一瓶水 = buy one bottle of water
• 买两杯水 = buy two cups of water
• 买那个牌子的水 = buy that brand of water

Because this sentence is talking about the action in general, 买水 is enough.

要 is flexible, and its exact meaning depends on context.

In this sentence, because of 马上, it suggests something fairly strong and immediate. It could be understood as:

• need to buy water right away
• have to buy water right away
• am going to buy water right away

The best interpretation is probably somewhere between need to and intend to.

That is why 要 often feels stronger and more definite than 想.

Yes, that translation is close, but it slightly weakens the force of 要.

A more accurate version would be something like:

• I not only want to go to the supermarket, but also need to buy water right away.

or

• I not only want to go to the supermarket, but I also have to buy water immediately.

If you translate 要 simply as want to, you may lose the stronger, more definite feeling.

而且 can definitely be used by itself.

By itself, it means something like:

• moreover
• and also
• besides

Example:

• 他很聪明，而且很努力。 = He is very smart, and also very hardworking.

When paired with 不但, it becomes the fuller pattern:

• 不但……而且…… = not only..., but also...

So 而且 is both an independent connector and part of this common paired structure.

Yes. 不但 and 不仅 are very similar and often interchangeable.

For example:

• 我不但想去超市，而且要马上买水。
• 我不仅想去超市，而且要马上买水。

Both are natural.

Very generally:

• 不仅 can sound a little more formal in some contexts
• 不但 is very common in both speech and writing

But for most learners, they can be treated as near-equivalents in this pattern.

The comma separates the two halves of the 不但……而且…… structure:

• 我不但想去超市，
• 而且要马上买水。

In written Chinese, this kind of comma is very common when connecting two balanced ideas. It helps readability and mirrors the natural pause in speech.

It is not optional in careful writing here; it is the normal punctuation.

Both can exist, but they are not exactly the same in emphasis.

• 要马上买水 focuses on the action of buying water immediately
• 马上要买水 can sound more like will soon buy water or is about to buy water, depending on context

In your sentence, 要马上买水 is the more straightforward way to say need to buy water right away.

So the pattern here is:

• 要 + 马上 + 买水

which is very natural.

Yes, it is grammatical and natural.

A native speaker might also say slightly different versions depending on tone and context, such as:

• 我不但想去超市，而且还要马上买水。
• 我不仅想去超市，还得马上买水。
• 我不但想去超市，还要立刻买水。

But your original sentence is absolutely normal Mandarin.