Η απαλλαγή είναι μεγάλη.

Η is the definite article for a feminine singular noun in the nominative case. In this sentence, it means the.

So:

• Η απαλλαγή = the exemption / the relief / the exemption from something
  (depending on context)

Greek normally uses articles very often, sometimes more often than English does, so seeing Η here is completely natural.

In Greek, every noun has a grammatical gender: masculine, feminine, or neuter.
Απαλλαγή is simply a feminine noun, and that is something you usually learn together with the word.

That is why it takes:

• the feminine article η
• the feminine adjective form μεγάλη

So the grammar matches like this:

• Η → feminine singular
• απαλλαγή → feminine singular noun
• μεγάλη → feminine singular adjective

Greek adjectives must agree with the noun they describe in:

• gender
• number
• case

Since απαλλαγή is feminine singular nominative, the adjective must also be feminine singular nominative.

The forms are:

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

So:

• Η απαλλαγή είναι μεγάλη. = correct

είναι means is and is the 3rd person singular form of the verb είμαι (to be).

So the structure is:

• Η απαλλαγή = subject
• είναι = verb
• μεγάλη = predicate adjective

This is very similar to English:

• The exemption is large.

Unlike some languages, Greek normally does use the verb to be in sentences like this.

Because here μεγάλη is not used as a simple noun modifier inside the noun phrase. It is a predicate adjective after είναι.

Compare:

• Η απαλλαγή είναι μεγάλη.
  = The exemption is large.

• Η μεγάλη απαλλαγή
  = the large exemption

So:

• before the noun: adjective directly describes the noun inside the phrase
• after είναι: adjective is part of the statement about the subject

Both are in the nominative singular.

Why?

• απαλλαγή is the subject of the sentence
• μεγάλη is a predicate adjective, and in Greek it agrees with the subject

So even though μεγάλη comes after the verb, it is still nominative because it refers back to the subject.

A rough pronunciation guide is:

• Η = ee
• απαλλαγή = a-pa-la-GHEE
• είναι = EE-ne
• μεγάλη = me-GHA-lee

A few helpful notes:

• η sounds like ee
• γ before η / ι / ε / αι is a softer sound, often approximated for English speakers as a y-like or soft gh sound
• the accent marks show where the stress falls:
  • απαλλαγή
  • είναι
  • μεγάλη

Greek uses accent marks to show which syllable is stressed.

In this sentence:

• απαλλαγή → stress on the last syllable
• είναι → stress on the first syllable
• μεγάλη → stress on the second syllable

Stress matters in Greek pronunciation, so it is important to learn words with their accents.

Yes, Greek word order is more flexible than English, though the most neutral version here is:

• Η απαλλαγή είναι μεγάλη.

You may also see:

• Μεγάλη είναι η απαλλαγή.

This can sound more emphatic, something like:

• It is the exemption that is large
• or The exemption is large

So the basic meaning stays the same, but the emphasis changes.

In the plural, all the words would change to match:

• Οι απαλλαγές είναι μεγάλες.
  = The exemptions are large.

Breakdown:

• Οι = feminine plural article
• απαλλαγές = plural noun
• είναι = still are/is form used for both singular and plural in the 3rd person
• μεγάλες = feminine plural adjective

Yes. Μια means a/an for a feminine singular noun.

So:

• Μια απαλλαγή είναι μεγάλη.
  = An exemption is large.

Grammatically it is correct. Whether it sounds natural depends on context, but the structure is fine.

This is a useful comparison:

• Η απαλλαγή = the exemption
• Μια απαλλαγή = an exemption

Not always. Like English big or great, μεγάλη can be literal or more abstract depending on context.

It can mean things like:

• large
• big
• great
• significant

So in a sentence like this, the exact natural English translation depends on what απαλλαγή refers to in context.