Το διάφραγμα είναι ανοιχτό.

Το is the definite article, equivalent to the in English.

In Το διάφραγμα είναι ανοιχτό, it is neuter singular nominative because διάφραγμα is a neuter singular noun and it is the subject of the sentence.

In Greek, every noun has a grammatical gender: masculine, feminine, or neuter.
Διάφραγμα is simply a neuter noun, and you have to learn that as part of the word.

Its ending helps too: many Greek neuter nouns end in -μα, for example:

• το γράμμα
• το όνομα
• το πρόβλημα
• το διάφραγμα

So when you see -μα, it is often a clue that the noun is neuter.

Είναι is the 3rd person singular form of είμαι, meaning is.

So the structure is:

• Το διάφραγμα = the subject
• είναι = is
• ανοιχτό = open

A few useful forms of είμαι are:

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

Because adjectives in Greek must agree with the noun they describe in gender, number, and case.

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

• masculine: ανοιχτός
• feminine: ανοιχτή
• neuter: ανοιχτό

So Το διάφραγμα είναι ανοιχτό is grammatically matched.

Yes. Here ανοιχτό is functioning as a predicate adjective, linked to the subject by είναι.

That is exactly like English sentences such as:

• The door is open
• The window is closed

In Greek, this is a very common pattern:

• Το παράθυρο είναι ανοιχτό
• Η πόρτα είναι κλειστή

In Modern Greek, the accent mark shows which syllable is stressed.

So:

• διάφραγμα → stress on διά
• είναι → stress on εί
• ανοιχτό → stress on -χτό

Unlike English, Greek spelling normally tells you where the stress goes, so the accent mark is very important.

This word order is the most neutral and natural.

But Greek word order is more flexible than English, so you may also hear other orders for emphasis, for example:

• Ανοιχτό είναι το διάφραγμα

That sounds more marked, as if the speaker is emphasizing open or contrasting it with something else. For a learner, Το διάφραγμα είναι ανοιχτό is the safest normal pattern.

Normally, no. In an ordinary sentence, if you mean the diaphragm, you would usually keep the article: το διάφραγμα.

Greek uses the definite article more often than English in many situations. A bare noun like διάφραγμα without το would usually sound unnatural here unless it appeared in a very special context, such as a heading, note, or technical label.

Yes, both mean open.

• ανοιχτό is very common in everyday Modern Greek
• ανοικτό is also correct, and may sound a bit more formal or conservative in some contexts

So you may see either:

• Το διάφραγμα είναι ανοιχτό
• Το διάφραγμα είναι ανοικτό

Both are acceptable.

Yes. Since διάφραγμα ends in -μα, it belongs to a very common neuter pattern.

A useful thing to remember is that many nouns of this type form their plural in -ματα:

• το διάφραγμα → τα διαφράγματα

You do not need that plural to understand this sentence, but it is a helpful pattern to learn early because many common Greek nouns work the same way.