Ο αριθμός πλαισίου είναι μακρύς.

Ο is the masculine singular nominative definite article, meaning the.

It is used because αριθμός is:

• masculine
• singular
• the subject of the sentence

So:

• ο αριθμός = the number

Greek articles change for gender, number, and case, unlike English the, which always stays the same.

Αριθμός is the nominative singular form, which is the form normally used for the subject of a sentence.

Here, ο αριθμός πλαισίου is the subject, and είναι μακρύς tells us something about it.

Dictionary form:

• αριθμός = number

Because it is a masculine noun in the nominative singular, it appears as ο αριθμός.

Because πλαισίου is in the genitive singular.

The basic form is:

• το πλαίσιο = the frame / chassis

But in this sentence Greek uses:

• αριθμός πλαισίου
  literally: number of frame/chassis

So πλαισίου shows a relationship like of the chassis or chassis- in English.

This is very common in Greek:

• ο αριθμός τηλεφώνου = telephone number
• η πόρτα του σπιτιού = the door of the house

So πλαισίου is there because Greek often uses the genitive where English uses a noun+noun combination.

Greek does not always need an article in this kind of noun phrase.

Here, πλαισίου is acting like a qualifier:

• ο αριθμός πλαισίου = the chassis/frame number

This is similar to English, where we often say:

• car door
• phone number
• train station

instead of always saying the door of the car, the number of the phone, etc.

If you said ο αριθμός του πλαισίου, that would sound more like the number of the chassis/frame, with a more explicit of the... structure. The version without the article is very natural for this kind of fixed expression.

Because it agrees with αριθμός, not with πλαισίου.

In Greek, adjectives must agree with the noun they describe in:

• gender
• number
• case

The noun being described is ο αριθμός, which is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That is why we get:

• μακρύς

Compare:

• ο αριθμός είναι μακρύς = masculine
• η λέξη είναι μακριά = feminine
• το όνομα είναι μακρύ = neuter

Μακρύς is the normal Modern Greek adjective meaning long.

An older or more formal form μακρός also exists, but in everyday Modern Greek μακρύς is the usual word.

So for a learner, the important thing is:

• μακρύς / μακριά / μακρύ = long

Examples:

• μακρύς δρόμος = long road
• μακριά φούστα = long skirt
• μακρύ όνομα = long name

Είναι is the 3rd person singular form of είμαι = to be.

Here it matches the subject:

• ο αριθμός = the number = it
• so: it is = είναι

With είμαι, the present-tense forms are:

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

Also:

• είμαστε / είμασταν? No — in the present standard form: είμαστε = we are
• είστε = you are
• είναι = they are

So είναι can mean both is and are, depending on the subject.

The word order here is the most neutral and straightforward:

• Ο αριθμός πλαισίου είναι μακρύς.
• subject + verb + adjective

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

• Μακρύς είναι ο αριθμός πλαισίου.

But that sounds more marked or emphatic. For a learner, the sentence as given is the normal pattern to follow.

A rough pronunciation is:

O a-riTH-MOS ple-see-OO EE-ne ma-KREES

A few notes:

• θ sounds like th in think
• αι sounds like e in met
• υ here sounds like ee
• the stressed syllables are:
  • αριθμός
  • πλαισίου
  • είναι
  • μακρύς

A more careful breakdown:

• Ο = o
• αριθμός = a-rith-MOS
• πλαισίου = ple-see-OO
• είναι = EE-ne
• μακρύς = ma-KREES

The dictionary forms are:

• ο αριθμός = number
• το πλαίσιο = frame, chassis
• είμαι = to be
• μακρύς, μακριά, μακρύ = long

In the sentence, some of those appear in changed forms because of grammar:

• πλαίσιο → πλαισίου because it is genitive singular
• μακρύς stays in its masculine singular nominative form because it matches αριθμός