Потом можно открыть воду.

Because открыть is a transitive verb here, so it takes a direct object in the accusative.

• nominative: вода
• accusative: воду

In this sentence, воду means something like the water supply / the tap water as the thing being turned on.

Открыть is the perfective form, which presents the action as a single completed event: to open / turn on.

In this sentence, the idea is that after some earlier step, it will be possible to do this one action: then you can turn on the water.

Compare:

• можно открыть воду = it is possible to turn on the water (one completed action)
• можно открывать воду = it is possible to start turning the water on / to be turning it on, which is less natural here

So открыть is the normal choice.

Можно means it is possible, one may, or you can depending on context.

It does not name a specific subject. Russian often leaves the subject vague in sentences like this. So можно открыть воду can mean:

• you can turn on the water
• it is possible to turn on the water now
• one may turn on the water

Which English version sounds best depends on context.

Because Russian often uses impersonal constructions where English uses you.

Можно + infinitive is a very common pattern:

• Можно войти. = You may come in. / It is possible to enter.
• Можно начинать. = You can start.

So Потом можно открыть воду does not need ты or вы. The sentence is about permission or possibility, not about emphasizing who does it.

Потом means then, afterward, or later.

At the beginning, it sets the time frame for the whole sentence: Потом можно открыть воду = After that, you can turn on the water.

Russian word order is flexible, so потом is often placed first when it gives the background timing for what follows.

Yes. Russian word order is flexible, and the meaning stays basically the same, though the emphasis changes.

For example:

• Потом можно открыть воду. = neutral; first sets up afterward
• Можно потом открыть воду. = emphasizes that doing it later is allowed/possible
• Открыть воду потом можно. = marked; contrasts with doing it now

The most natural neutral version here is the original: Потом можно открыть воду.

Yes, it is a normal everyday expression, especially in the sense of turn on the water.

Russian often uses открыть and закрыть with things like taps, valves, and flows:

• открыть кран = open the tap
• закрыть кран = close the tap
• открыть воду = turn on the water
• закрыть воду = turn off the water

In very literal terms, Russian thinks of this as opening access to the water.

Because открыть воду is the usual expression when talking about turning on water from a tap, faucet, or valve.

Включить is more natural for things like:

• включить свет = turn on the light
• включить телевизор = turn on the TV

With water, включить воду can appear in some contexts, especially when talking about restoring a water supply in a building or district, but for an ordinary tap/faucet situation, открыть воду is more natural.

Yes, and that would be very natural too.

There is a small difference in focus:

• открыть воду focuses on the water flow
• открыть кран focuses on the tap/faucet itself

In practice, both can often fit the same situation.

These words express different ideas:

• можно = it is possible / allowed
• нужно = it is necessary
• надо = need to / must; often similar to нужно

So:

• Потом можно открыть воду. = Then you can turn on the water.
• Потом нужно открыть воду. = Then you need to turn on the water.

That is an important distinction: можно is about permission or possibility, not obligation.

The main word stresses are:

• пото́м
• мо́жно
• откры́ть
• во́ду

So the sentence is pronounced roughly: пото́м мо́жно откры́ть во́ду

For an English speaker, it is useful to notice that ё is not involved here, so nothing unusual is hidden in the spelling. The stress just has to be learned word by word.

It is neutral everyday Russian.

Nothing in it is especially formal or informal. It could be used:

• in instructions
• in conversation
• in practical household situations
• in repair or plumbing contexts

Its tone depends mostly on the surrounding context, not on the grammar itself.