Η ουσία είναι άγνωστη.

• Η = the definite article (the), feminine singular nominative
• ουσία = a noun, feminine singular nominative
• είναι = the verb is / are, from είμαι (to be)
• άγνωστη = an adjective, feminine singular nominative, meaning unknown

So the structure is:

article + noun + verb + adjective
= The essence/substance is unknown

Because ουσία is a feminine noun.

In Greek, the definite article changes according to gender, number, and case:

• ο = masculine singular nominative
• η = feminine singular nominative
• το = neuter singular nominative

Since ουσία is feminine singular, it takes η:

• η ουσία = the essence / the substance

Because it describes ουσία, and Greek adjectives usually agree with the noun they describe in:

• gender
• number
• case

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

• masculine: άγνωστος
• feminine: άγνωστη
• neuter: άγνωστο

So:

• η ουσία είναι άγνωστη
• not η ουσία είναι άγνωστος
• not η ουσία είναι άγνωστο

Here άγνωστη is not just modifying the noun inside a noun phrase. It is functioning as a predicate adjective after the verb είναι.

Compare:

• η άγνωστη ουσία = the unknown substance

  • here the adjective is directly attached to the noun

• η ουσία είναι άγνωστη = the substance is unknown

  • here the adjective comes after is

This works much like English:

• the unknown substance
• the substance is unknown

είναι is the present tense form of είμαι (to be).

In this sentence it means is.

A useful thing to know: είναι is used for both:

• he/she/it is
• they are

The subject tells you which one is meant.

Here the subject is singular (η ουσία), so it means is.

Greek words normally show stress with an accent mark.

• είναι is stressed on the first syllable: EÍ-ne
• ουσία is stressed on the last meaningful syllable: ou-SÍ-a
• άγνωστη is stressed on the first syllable: Á-gno-sti

So the accents help you pronounce the words correctly.

A reasonable pronunciation guide is:

ee oo-SEE-ah EE-ne AH-ghno-stee

More closely:

• Η = ee
• ουσία = oo-SEE-ah
• είναι = EE-ne
• άγνωστη = AH-ghno-stee

Notes:

• ου sounds like oo in food
• γν in άγνωστη has a voiced sound, roughly ghn
• Greek η, ι, υ, ει, οι can all sound like ee

ουσία can mean several related things depending on context, such as:

• essence
• substance
• core nature
• sometimes even substance in a philosophical or technical sense

So the best English translation depends on context.

For example:

• in philosophy: essence or substance
• in chemistry or technical writing: often substance
• in a more abstract discussion: essence

Usually not in this sentence if you mean the essence / the substance as a definite subject.

Greek often uses the definite article where English does, and many nouns sound incomplete without it in ordinary statements.

So:

• Η ουσία είναι άγνωστη. = natural, standard
• Ουσία είναι άγνωστη. = not natural in normal Greek for this meaning

If you wanted a more general or different meaning, Greek would usually express that in some other way, not simply by dropping the article here.

Yes. Greek word order is more flexible than English, although the basic order here is very natural.

Standard:

• Η ουσία είναι άγνωστη.

You could also say:

• Άγνωστη είναι η ουσία.

This changes the emphasis, not the core meaning.
The second version sounds more marked, as if stressing unknown.

English does this less freely, so learners should usually stick with the standard order first.

No. άγνωστη is only the feminine singular form.

The full adjective changes form:

• άγνωστος = masculine singular
• άγνωστη = feminine singular
• άγνωστο = neuter singular

Examples:

• Ο άντρας είναι άγνωστος. = The man is unknown.
• Η ουσία είναι άγνωστη. = The substance is unknown.
• Το όνομα είναι άγνωστο. = The name is unknown.

Yes. In a sentence like this, the subject is in the nominative, and the predicate adjective also appears in the nominative to agree with the subject.

So:

• η ουσία = nominative subject
• άγνωστη = nominative adjective agreeing with ουσία

This is similar to older grammar terminology in some other languages, where after to be, you often get a subject complement rather than an object.

Yes. Without more context, ουσία can support both translations.

A learner should not assume there is always just one fixed English equivalent. Greek words often cover a range that overlaps with several English words.

So if you see:

• Η ουσία είναι άγνωστη.

the intended English could be:

• The essence is unknown.
• The substance is unknown.

Context decides which is best.