Η φωτεινότητα είναι χαμηλή.

Η is the definite article meaning the. It matches φωτεινότητα, which is a feminine singular noun.

So:

• Η φωτεινότητα = the brightness

Greek uses the definite article more often than English, especially with abstract nouns, so Η φωτεινότητα is very natural.

That is a very common question. In Greek, the definite article is often used with nouns in a general or abstract sense.

So Greek says:

• Η φωτεινότητα είναι χαμηλή.

where English may say either:

• The brightness is low
• Brightness is low

Greek strongly prefers the article here, while English is more flexible.

Φωτεινότητα is a noun. Grammatically, it is:

• feminine
• singular
• nominative case here, because it is the subject of the sentence

It means brightness.
The ending -ότητα is common in abstract nouns, similar to English endings like -ness or -ity.

Because it describes φωτεινότητα, and Greek adjectives must agree with the noun they describe in:

• gender
• number
• case

Since φωτεινότητα is feminine singular nominative, the adjective must also be feminine singular nominative:

• masculine: χαμηλός
• feminine: χαμηλή
• neuter: χαμηλό

So χαμηλή is the correct form here.

Είναι is the present tense form of the verb είμαι, meaning to be.

Here it means is.

So the sentence structure is:

• Η φωτεινότητα = the subject
• είναι = is
• χαμηλή = low

A useful thing to know is that είναι is used for he/she/it is and also they are, depending on context.

It is in the nominative, just like φωτεινότητα.

That may feel unusual if you are thinking in terms of English grammar, but in Greek, after είμαι (to be), the adjective that describes the subject normally agrees with it and stays in the nominative.

So:

• Η φωτεινότητα = nominative subject
• χαμηλή = nominative predicate adjective

A rough pronunciation is:

• ee fo-ti-NO-ti-ta EE-ne ha-mi-LEE

A more careful breakdown:

• Η = ee
• φωτεινότητα = fo-ti-NO-ti-ta
• είναι = EE-ne
• χαμηλή = ha-mi-LEE

A few key pronunciation points:

• φ sounds like f
• αι in είναι sounds like e
• η, ι, υ, ει, οι often sound like ee
• χ is a throaty sound, somewhat like the ch in German Bach, but before α it may sound closer to a stronger h/kh

The accent marks show which syllable is stressed.

So:

• φωτεινότητα
• είναι
• χαμηλή

Stress matters in Greek pronunciation, and it is normally written, so learners should pay close attention to it.

The most neutral word order here is:

• Η φωτεινότητα είναι χαμηλή.

This is the standard subject + verb + adjective order.

Greek word order is more flexible than English, so other orders are possible for emphasis, for example:

• Χαμηλή είναι η φωτεινότητα.

This could sound more marked or emphatic, something like It is low, the brightness or The brightness is low with emphasis on low.

For normal learning and everyday use, the original order is the best one to remember.

In a normal full sentence like this, no, you should keep είναι.

Greek usually does not omit the verb to be in ordinary statements the way some other languages do.

So:

• Η φωτεινότητα είναι χαμηλή. = correct

Leaving out είναι would sound incomplete in standard speech, except perhaps in special contexts like notes, labels, headlines, or very informal shorthand.

Not with φωτεινότητα, because the adjective has to match the noun.

Since φωτεινότητα is feminine singular, you need:

• χαμηλή

You would use the other forms with different nouns:

• Ο ήχος είναι χαμηλός. = The sound is low.
  (ήχος is masculine)
• Το επίπεδο είναι χαμηλό. = The level is low.
  (επίπεδο is neuter)

So in your sentence, χαμηλή is the only correct choice.