Карниз над окном старый, но крепкий.

In Russian, the verb to be is usually omitted in the present tense.

So:

• Карниз над окном старый, но крепкий. literally looks like Cornice above the window old, but sturdy
• but it naturally means The curtain rod/cornice above the window is old but sturdy.

If you wanted past or future, Russian would use a form of быть:

• Карниз был старый, но крепкий. = The cornice was old but sturdy.
• Карниз будет старый, но крепкий. = The cornice will be old but sturdy.

Карниз can mean a cornice, ledge, or curtain rod, depending on context.

In a sentence with над окном (above the window), many learners will picture either:

• an architectural cornice/molding above the window, or
• a curtain rod

So the exact English translation depends on the situation.

Because the preposition над usually takes the instrumental case when it means above/over in a location sense.

The noun окно is neuter, and its instrumental singular form is:

• окно → окном

So:

• над окном = above the window

This is a very common pattern:

• над столом = above the table
• над дверью = above the door
• над рекой = above the river

Because they agree with карниз, which is:

• masculine
• singular
• nominative

So the predicate adjectives also appear in the masculine singular nominative form:

• старый = old
• крепкий = sturdy/strong/solid

If the noun changed, the adjective endings would change too:

• рама старая, но крепкая = the frame is old but sturdy
• окно старое, но крепкое = the window is old but sturdy
• карнизы старые, но крепкие = the cornices are old but sturdy

Yes, крепкий often means strong, sturdy, solid, or durable, depending on context.

For a physical object like карниз, it suggests that it is:

• solid
• well-built
• not weak or fragile

So in English, sturdy is often the best fit here.

Be aware that крепкий is used in many contexts:

• крепкий чай = strong tea
• крепкий человек = a tough/strong person
• крепкий дом = a solidly built house

Because но means but, and in Russian it normally joins two contrasting parts of a sentence, so a comma is used before it.

Here the contrast is:

• старый = old
• но крепкий = but sturdy

So:

• старый, но крепкий = old, but sturdy

This is standard Russian punctuation.

Yes, you could change the word order, although the original sentence is the most neutral.

Original:

• Карниз над окном старый, но крепкий.

Other possible versions:

• Старый, но крепкий карниз над окном.
  This sounds more like a noun phrase: The old but sturdy cornice above the window
• Над окном карниз старый, но крепкий.
  This shifts attention to the location first: Above the window, the cornice is old but sturdy.

Russian word order is flexible, but different orders change emphasis and style.

Primarily, над окном describes карниз.

So the meaning is:

• The cornice that is above the window is old but sturdy.

In other words, над окном tells you which cornice we are talking about.

Structurally, it works like an adverbial location phrase, but in meaning it helps identify the noun.

Yes, sometimes Russian uses a dash in sentences like this for emphasis:

• Карниз над окном — старый, но крепкий.

The dash can make the sentence feel a little more formal, marked, or expressive. But without the dash:

• Карниз над окном старый, но крепкий.

is completely normal and natural.

From the form and the meaning.

• окном = instrumental singular of окно
• instrumental plural would be окнами

So:

• над окном = above the window
• над окнами = above the windows

Yes. In Russian, старый can mean:

• old in age
• worn from use
• long-standing
• not new

In this sentence, it most likely means simply that the cornice is not new or has been there a long time.

Because it is paired with но крепкий (but sturdy), the sentence suggests:

• it may be old,
• but it is still solid and reliable.

It is neutral and natural Russian.

It does not sound especially formal or especially casual. You could easily see or hear it in:

• everyday speech
• a description in writing
• a practical comment about a room or building

So it is a good example of standard Russian usage.