Gaz vanası kapalı; bununla birlikte su vanası hâlâ açık.

Because gaz vanası and su vanası are Turkish noun compounds.

In Turkish, when one noun describes another like this, the second noun usually takes the 3rd person possessive suffix:

• gaz vana ❌
• gaz vanası ✅ = gas valve
• su vanası = water valve

So vanası literally looks like its valve, but in this kind of compound it simply means the valve of gas / water, or more naturally in English, gas valve / water valve.

This is a very common pattern:

• araba kapısı = car door
• ev sahibi = homeowner / owner of the house
• mutfak masası = kitchen table

In Turkish, the verb to be is often omitted in the present tense.

So:

• Gaz vanası kapalı. = The gas valve is closed.
• Su vanası açık. = The water valve is open.

There is no separate word for is here. Turkish often just puts the noun and the adjective/state word together.

You could think of the structure as:

• [subject] + [state/adjective]

This is completely normal and very common in everyday Turkish.

Bununla birlikte means however, nevertheless, even so, or at the same time / along with this, depending on context.

In this sentence, it works like a connector between two ideas:

• Gaz vanası kapalı; bununla birlikte su vanası hâlâ açık.

A natural English sense is:

• The gas valve is closed; however, the water valve is still open.

Literally, bununla birlikte breaks down as:

• bu = this
• bununla = with this
• birlikte = together

So the literal sense is something like together with this or with that being the case, which developed into a linking expression like however / nevertheless.

It is more formal or written-sounding than ama.

Because ile often combines with the previous word.

• bu + ile → bununla

This is the normal fused form here.

A few related examples:

• benimle = with me
• seninle = with you
• onunla = with him/her/it
• bununla = with this
• şununla = with that
• neyle = with what

So bununla birlikte is a fixed, very natural expression.

Hâlâ means still.

So:

• su vanası hâlâ açık = the water valve is still open

It shows that the condition continues.

Compare:

• Su vanası açık. = The water valve is open.
• Su vanası hâlâ açık. = The water valve is still open.

The second one suggests persistence: maybe someone expected it to be closed, but it remains open.

The circumflex helps distinguish hâlâ from hala.

• hâlâ = still
• hala = paternal aunt

In modern everyday writing, many people skip the circumflex, but in careful standard spelling it is useful here because it prevents confusion.

So in this sentence, hâlâ definitely means still.

It depends on the object.

With something like a valve, door, window, or box, açık usually means open and kapalı means closed.

So here:

• gaz vanası kapalı = the gas valve is closed
• su vanası açık = the water valve is open

In other contexts, these words can overlap with English on/off:

• Işık açık. = The light is on.
• Dükkan kapalı. = The shop is closed.
• Televizyon açık. = The TV is on.

So the basic idea is a state, and English translation depends on what thing is being described.

Yes. Açık and kapalı describe the current state of the valves.

This is different from a verb meaning to open or to close.

Compare:

• Gaz vanası kapalı. = The gas valve is closed.
  State.

• Gaz vanasını kapattı. = He/she closed the gas valve.
  Action.

• Su vanasını açtı. = He/she opened the water valve.
  Action.

So in your sentence, the focus is not on who did it, but on the present condition.

The semicolon links two closely related complete clauses:

• Gaz vanası kapalı
• bununla birlikte su vanası hâlâ açık

It shows a stronger pause than a comma, but a closer connection than a full stop.

You could also write this with a full stop:

• Gaz vanası kapalı. Bununla birlikte su vanası hâlâ açık.

That would also be correct.

The semicolon is mainly a punctuation/style choice here.

Yes, in many contexts you could.

For example:

• Gaz vanası kapalı ama su vanası hâlâ açık.

This is more everyday and conversational.

The version with bununla birlikte sounds more formal, written, or explanatory:

• Gaz vanası kapalı; bununla birlikte su vanası hâlâ açık.

So the meaning is similar, but the tone is different.

Yes, Turkish word order is flexible, though some versions sound more natural than others.

The original sentence is very natural:

• Gaz vanası kapalı; bununla birlikte su vanası hâlâ açık.

You could also say:

• Gaz vanası kapalı; bununla birlikte hâlâ su vanası açık.

That is possible, but slightly less neutral.

Because hâlâ modifies the state açık, the original placement feels smooth and clear:

• su vanası hâlâ açık

In general, Turkish often places words like hâlâ before the adjective or predicate they modify.

Because both are the subjects of their clauses, not direct objects.

• Gaz vanası kapalı.
• Su vanası hâlâ açık.

These are subject + predicate structures, so no accusative is needed.

You would use the accusative if the valve were the object of an action:

• Gaz vanasını kapattı. = He/she closed the gas valve.
• Su vanasını açtı. = He/she opened the water valve.

Notice the difference:

• vanası = the valve, as subject
• vanasını = the valve, as definite direct object

A natural rhythm would give a pause after kapalı, then continue with the contrast:

Gaz vanası kapalı; bununla birlikte su vanası hâlâ açık.

In speech, many speakers would probably choose a simpler connector such as ama unless they want a more formal tone.

So in everyday speech, you are more likely to hear:

Gaz vanası kapalı ama su vanası hâlâ açık.

But the original sentence is perfectly correct and natural in a more formal or written style.