Oświetlenie jest sprawne.

• oświetlenie = a noun, lighting / illumination
• jest = the 3rd person singular of być, to be
• sprawne = an adjective meaning working, functional, in good condition

So the structure is basically:

[noun] + [is] + [adjective]

Because it has to agree with oświetlenie.

Oświetlenie is:

• singular
• neuter
• nominative

So the adjective also has to be:

• singular
• neuter
• nominative

That gives sprawne.

Compare:

• samochód jest sprawny = the car is working/functional
• instalacja jest sprawna = the installation is working
• oświetlenie jest sprawne = the lighting is working
• urządzenia są sprawne = the devices are working

It is neuter.

A very common clue is the ending -e or -ie in the dictionary form of a noun. Many Polish neuter nouns end this way, especially nouns like:

• jedzenie
• pytanie
• oświetlenie

So when you see oświetlenie, you should expect neuter agreement:

• to oświetlenie
• dobre oświetlenie
• oświetlenie jest sprawne

It is singular.

Even though in English we might think of lights as plural, Polish oświetlenie is usually a singular mass/collective noun meaning lighting as a system or overall illumination.

So:

• Oświetlenie jest sprawne = the lighting is functional

If you wanted actual individual lights, you might say:

• Światła są sprawne = the lights are working

Because Polish does not have articles like a or the.

So oświetlenie can mean:

• lighting
• the lighting

The exact meaning depends on context.

That is completely normal in Polish.

A practical approximation is:

oh-shvye-tle-NYE-nye

The stress is on the second-to-last syllable:

o-świe-tle-nie → tle

A few pronunciation notes:

• ś is a soft sh-like sound
• ł sounds like English w
• nie at the end sounds roughly like nye

You do not need a perfect English spelling approximation to be understood, but the main thing is to keep the stress on -le-.

Not exactly.

In this sentence, sprawne means something like:

• working
• functional
• in working order
• operational

For equipment, machines, systems, brakes, lighting, etc., this is a very common meaning.

Depending on context, sprawny / sprawna / sprawne can also mean:

• efficient
• capable
• physically fit/agile

But here, with oświetlenie, the meaning is clearly functional / working properly.

In a full normal sentence, Oświetlenie jest sprawne is the standard form.

You may see Oświetlenie sprawne in:

• checklists
• inspection reports
• labels
• shorthand notes
• technical forms

There it works like an abbreviated style, similar to writing Lighting: OK.

So:

• normal sentence: Oświetlenie jest sprawne
• technical shorthand: Oświetlenie sprawne

This is a very useful distinction.

• oświetlenie = lighting, the illumination system or general lighting
• światło = light
• światła = lights

Examples:

• Oświetlenie jest sprawne = the lighting system is functional
• Światło się świeci = the light is on
• Światła w samochodzie są sprawne = the car lights are working

So oświetlenie sounds a bit more technical or general, while światło/światła often refers to specific lights.

Here sprawne is in the nominative, because it is a predicate adjective after jest.

With być:

• predicate adjectives are normally in the nominative
• predicate nouns are often in the instrumental

Compare:

• On jest miły. = He is nice. → adjective, nominative
• On jest lekarzem. = He is a doctor. → noun, instrumental

So:

• Oświetlenie jest sprawne. → adjective, nominative

Yes, but it sounds a bit formal / technical / inspection-like.

You would often hear it in contexts like:

• a vehicle inspection
• a maintenance report
• a building checklist
• a technical assessment

In more everyday speech, people might also say:

• Oświetlenie działa. = The lighting works.
• Światło działa. = The light works.
• Światła działają. = The lights work.

So Oświetlenie jest sprawne is natural, but slightly more formal and technical.

The most direct negative is:

Oświetlenie nie jest sprawne.
= The lighting is not functional.

You may also see:

Oświetlenie jest niesprawne.
= The lighting is faulty / not in working order.

Both are correct, but there is a slight difference:

• nie jest sprawne = simply negates is functional
• jest niesprawne = sounds a bit more fixed/technical: is faulty

You just change jest:

• Oświetlenie było sprawne. = The lighting was functional.
• Oświetlenie będzie sprawne. = The lighting will be functional.

Notice that sprawne stays the same, because oświetlenie is still neuter singular.