suidou ga mada tukaenai node, suupaa de mizu wo kaimasita.

Here, 水道（すいどう） means the water supply / running water / plumbing service.

It does not mean the water itself.
That is why the sentence later says 水を買いました — I bought water.

So the contrast is:

• 水道 = the water system / running water at home
• 水 = water as a thing you drink or use

In this sentence, 水道がまだ使えない means something like:

• the running water still isn't usable
• the water service still isn't working

が marks 水道 as the thing being described by 使えない.

So:

• 水道が使えない = the water supply can't be used
• literally, as for the water supply, it is unusable

This is a common pattern:

• X が 使える = X can be used
• X が 使えない = X cannot be used

So が is natural here because 水道 is the thing that has the property usable / not usable.

With a negative verb, まだ usually means still.

So:

• まだ使えない = still can't use it
• まだ来ていない = hasn't come yet
• まだ終わっていない = isn't finished yet

This is different from English, where yet often appears in negative sentences. In Japanese, まだ often covers that idea.

So 水道がまだ使えない means:

• the water still isn't usable
• the water service isn't working yet

使えない is the negative form of 使える.

The chain is:

• dictionary form: 使う = to use
• potential form: 使える = can use / is usable
• negative potential: 使えない = cannot use / is not usable

So this sentence is not just saying someone doesn't use the water supply.
It is saying the water supply cannot be used.

That is why the translation is about the water not working, not about someone choosing not to use it.

使えない focuses on not being able to use the water system.

Different expressions give slightly different nuances:

• 水道が使えない
  = the water supply can't be used / isn't available

• 水が出ない
  = water doesn't come out

• 水道がない
  = there is no water supply / no plumbing

In real life, 水が出ない is also very common if you want to say no water is coming from the tap.
But 水道が使えない is natural when talking more generally about the service being unavailable.

ので means because or so.

So:

• 水道がまだ使えないので、スーパーで水を買いました。
• Because the water still wasn't usable, I bought water at the supermarket.

Compared with から, ので often sounds:

• a little softer
• a little more explanatory
• a little more neutral or polite

Very roughly:

• から = because, often more direct
• ので = because/since, often more gentle or explanatory

Both would work here, but ので sounds very natural.

The comma marks a pause after the reason clause:

• 水道がまだ使えないので、 = because the water still wasn't usable,
• スーパーで水を買いました。 = I bought water at the supermarket.

Japanese commas are often used more flexibly than English commas.
This comma helps the reader see the sentence structure clearly, but it is not always absolutely required.

で marks the place where an action happens.

So:

• スーパーで買いました = bought it at the supermarket

This is different from に, which often marks:

• destination
• existence
• specific time
• indirect object

Since buying happens at the supermarket, で is the correct particle here.

を marks the direct object of the verb 買いました.

So:

• 水を買いました = bought water

Here:

• 水 = the thing bought
• 買いました = bought

This is one of the most basic uses of を.

Because the sentence is talking about a completed action:

• the water supply was still unusable
• so the speaker bought water

買いました is the polite past form of 買う.

• 買う = buy
• 買います = buy / will buy
• 買いました = bought

Even though the cause clause contains a present-form-style adjective/verb (使えない), the main action happened in the past, so 買いました is used.

Japanese often omits subjects when they are obvious from context.

So even though English usually says:

• I bought water

Japanese often just says:

• 水を買いました

The listener normally understands who did it from the situation.

If needed, you could say:

• 私はスーパーで水を買いました。

But in many everyday sentences, leaving out 私は sounds more natural.

Yes. スーパー is short for スーパーマーケット and usually means supermarket or sometimes grocery store, depending on context.

This kind of shortened loanword is very common in Japanese.

So:

• スーパーで水を買いました = I bought water at the supermarket

Here are the main readings:

• 水道 = すいどう
• が = ga
• まだ = mada
• 使えない = つかえない
• ので = node
• スーパー = sūpā
• で = de
• 水 = みず
• を = o (pronounced like お)
• 買いました = かいました

Full reading:

• すいどう が まだ つかえない ので、スーパー で みず を かいました。

Yes. A very natural English translation would be:

• Because the water was still out, I bought water at the supermarket.
• Since we still couldn't use the running water, I bought water at the supermarket.
• The water still wasn't working, so I bought water at the supermarket.

A word-for-word breakdown is useful for study, but natural English often changes the wording a bit.