Η συλλογή είναι μεγάλη.

It is a simple subject + verb + adjective sentence:

• Η συλλογή = the subject
• είναι = the verb to be
• μεγάλη = an adjective describing the subject

So grammatically, it works very much like English The collection is large.

Because συλλογή is a feminine singular noun, and the Greek definite article has to match it.

In the nominative singular:

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

So Greek says η συλλογή because συλλογή is feminine.

Because grammatical gender in Greek is part of the noun itself. You learn the noun together with its article:

• η συλλογή

That tells you the noun is feminine. There is not always a logical reason why a thing is masculine, feminine, or neuter; it is just part of the word. That said, many nouns ending in -ή are feminine, so συλλογή fits a common pattern.

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

The adjective big / large has these basic singular forms:

• μεγάλος = masculine
• μεγάλη = feminine
• μεγάλο = neuter

Since συλλογή is feminine singular, the adjective must also be feminine singular:

• η συλλογή είναι μεγάλη

Because μεγάλη is being used as a predicate adjective after είναι.

Compare:

• Η συλλογή είναι μεγάλη = the collection is large
• Η μεγάλη συλλογή = the large collection

In the first sentence, μεγάλη is part of the predicate, so it does not need an article. In the second example, μεγάλη is directly attached to the noun, so it is part of the noun phrase.

Η συλλογή is in the nominative case because it is the subject of the sentence.

μεγάλη is also nominative, because predicate adjectives after είμαι normally agree with the subject in case, gender, and number.

So both the noun phrase and the adjective are:

• feminine
• singular
• nominative

Interestingly, είναι can mean both is and are in Modern Greek.

Examples:

• Η συλλογή είναι μεγάλη = the collection is large
• Οι συλλογές είναι μεγάλες = the collections are large

So the form είναι itself does not change between third-person singular and third-person plural. The subject tells you whether it means is or are.

You would usually use the indefinite article μια:

• Μια συλλογή είναι μεγάλη = a collection is large

So:

• η συλλογή = the collection
• μια συλλογή = a collection

The choice depends on whether you mean a specific collection or a non-specific one.

Yes, Greek word order is more flexible than English, although Η συλλογή είναι μεγάλη is the most neutral order.

For emphasis, Greek might also say:

• Μεγάλη είναι η συλλογή

That puts more focus on μεγάλη. But the basic learner-friendly order is the one you have here:

• Η συλλογή είναι μεγάλη

A rough English-friendly pronunciation is:

ee see-loh-YEE EE-ne meh-GHA-lee

A few helpful notes:

• Η sounds like ee
• συλλογή is stressed on the last syllable: -γή
• είναι is stressed on the first syllable
• μεγάλη is stressed on γά
• the double λλ is spelled double, but in normal Modern Greek pronunciation it is not held much longer than a single λ

This is only an approximation, but it is a good starting point.

The accent marks show which syllable is stressed.

In this sentence:

• συλλογή → stress on γή
• είναι → stress on εί
• μεγάλη → stress on γά

Stress matters in Greek, so the accent marks are important for both pronunciation and correct spelling.

It can mean different things depending on context.

Most commonly, μεγάλος / μεγάλη / μεγάλο means:

• big
• large

But in some contexts it can also mean:

• great
• important
• older or grown-up, depending on the noun

In Η συλλογή είναι μεγάλη, the natural meaning is large / big, because it is describing the size or extent of the collection.