Το προσκήνιο είναι φωτεινό.

Το is the definite article meaning the.

In this sentence, το προσκήνιο means the foreground.

Greek articles change for gender, number, and case. Here:

• το = the for neuter singular
• προσκήνιο is a neuter singular noun

So το προσκήνιο is the correct way to say the foreground.

You can tell from two clues:

1. It uses the article το, which is the neuter singular definite article.
2. The noun ends in -ιο, which is a very common neuter ending in Greek.

So in this sentence:

• το προσκήνιο = neuter singular

That matters because the adjective also has to match it.

Because Greek adjectives must agree with the noun they describe.

Since προσκήνιο is neuter singular, the adjective must also be neuter singular:

• masculine: φωτεινός
• feminine: φωτεινή
• neuter: φωτεινό

So:

• το προσκήνιο είναι φωτεινό = correct
  because both the noun and adjective are neuter singular.

Είναι means is here. It is the 3rd person singular form of the verb είμαι = to be.

So:

• είμαι = I am
• είσαι = you are
• είναι = he/she/it is

In this sentence:

• Το προσκήνιο είναι φωτεινό.
• The foreground is bright.

Φωτεινό is a predicate adjective. That means it comes after the verb to be and describes the subject.

So the structure is:

• Το προσκήνιο = subject
• είναι = verb
• φωτεινό = predicate adjective

This is like English:

• The foreground is bright.

Even though it comes after είναι, it still matches the subject in gender and number.

Because φωτεινό is being used as an adjective after είναι, not as a noun.

In Greek, when an adjective is used predicatively after to be, it usually does not take an article:

• Το προσκήνιο είναι φωτεινό.
• The foreground is bright.

If you put an article before it, the structure and meaning would change.

It is in the nominative case, because it is the subject of the sentence.

• Το προσκήνιο = the subject
• είναι = is
• φωτεινό = describes the subject

In sentences with είμαι (to be), the subject is nominative, and the predicate adjective agrees with it.

Yes. Greek word order is often more flexible than English.

The neutral, straightforward order is:

• Το προσκήνιο είναι φωτεινό.

But you could also say:

• Φωτεινό είναι το προσκήνιο.

That version puts more emphasis on φωτεινό.

Even though the order changes, the meaning stays basically the same. The default version for learners is the original one.

A simple learner-friendly pronunciation is:

• to pro-SKI-nio EE-ne fo-ti-NO

A few notes:

• ή/ί/ει/οι/υ often sound like ee
• the stress marks show which syllable is stressed:
  • προσκήνιο
  • είναι
  • φωτεινό

So the stressed syllables are:

• proSKInio
• EEne
• fotiNO

It can mean several closely related things, depending on context:

• bright
• light
• well-lit
• luminous sometimes, depending on style

In a sentence like this, The foreground is bright or The foreground is well-lit would both be natural translations.

So φωτεινό is broader than just one English word.

Not only. Προσκήνιο can mean:

• the foreground in an image or scene
• the front of the stage
• more figuratively, the forefront or spotlight

In this sentence, it most naturally means the foreground of a visual scene.

Normally, in standard modern Greek, you would keep είναι in a sentence like this:

• Το προσκήνιο είναι φωτεινό.

Greek does sometimes omit parts of a sentence in informal contexts, headlines, notes, or poetic language, but for normal full sentences, learners should use είναι.

So the standard form is:

• Το προσκήνιο είναι φωτεινό.

It is neutral standard Greek. There is nothing especially formal or especially casual about it.

It would sound natural in many contexts, for example:

• describing a photograph
• talking about a painting
• commenting on lighting in a scene

So it is a good model sentence for everyday standard Greek.