Η μεγαλύτερη δυσκολία είναι η προφορά.

A natural pronunciation is roughly:

ee me-gha-LEE-teh-ree thee-sko-LEE-a EE-ne ee pro-fo-RA

A few helpful notes:

• η sounds like ee
• γ before α sounds like a soft throaty gh
• δ in Modern Greek sounds like th in this
• The stressed syllables are marked by the accent:
  • μεγαλύτερη
  • δυσκολία
  • είναι
  • προφορά

So the main rhythm is:

ee me-gha-LEE-teh-ree thee-sko-LEE-a EE-ne ee pro-fo-RA

Word by word:

• Η = the
• μεγαλύτερη = bigger / biggest / greater / greatest depending on context
• δυσκολία = difficulty
• είναι = is
• η = the
• προφορά = pronunciation

So the whole sentence means The greatest difficulty is pronunciation or The biggest difficulty is the pronunciation.

Because Greek uses the definite article with both noun phrases here:

• η μεγαλύτερη δυσκολία = the biggest difficulty
• η προφορά = the pronunciation

In English, after is, you may or may not repeat the depending on style, but in Greek it is very normal to have the article on both sides of είναι when both things are definite.

Because it has to agree with δυσκολία.

δυσκολία is:

• feminine
• singular
• nominative

So the adjective also appears in the feminine singular nominative form:

• masculine: μεγαλύτερος
• feminine: μεγαλύτερη
• neuter: μεγαλύτερο

Greek adjectives change form to match the noun they describe.

Formally, μεγαλύτερη is the comparative form, meaning bigger / greater.

But in Greek, the comparative form with the definite article often expresses a superlative meaning:

• μεγαλύτερη δυσκολία = bigger difficulty
• η μεγαλύτερη δυσκολία = the biggest / greatest difficulty

So in this sentence it is understood as the greatest difficulty.

Yes. That is also correct.

Two common ways to express this idea are:

• η μεγαλύτερη δυσκολία
• η πιο μεγάλη δυσκολία

Both can mean the biggest difficulty.

Very generally:

• μεγαλύτερη is a single-word comparative form
• πιο μεγάλη uses πιο
  • adjective

Both are very common in Modern Greek.

Both are in the nominative.

Why?

• η μεγαλύτερη δυσκολία is the subject
• η προφορά comes after είναι and is a predicate noun

In Greek, with the verb είμαι (to be), both the subject and the predicate noun are usually in the nominative.

είναι is the 3rd person singular present of είμαι, meaning is.

So:

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

Here it matches the singular subject η μεγαλύτερη δυσκολία.

Because that is a very normal position for adjectives in Greek.

So:

• η μεγαλύτερη δυσκολία = the biggest difficulty

Greek adjectives can sometimes come after the noun too, but putting the adjective before the noun is extremely common, especially in everyday statements like this.

Yes. Greek word order is more flexible than English.

For example, you could also say:

• Η προφορά είναι η μεγαλύτερη δυσκολία.

This means the same thing: Pronunciation is the greatest difficulty.

The difference is mainly emphasis:

• Η μεγαλύτερη δυσκολία είναι η προφορά puts focus first on the greatest difficulty
• Η προφορά είναι η μεγαλύτερη δυσκολία puts focus first on pronunciation

In this sentence, η προφορά is the natural choice because it refers to pronunciation as a definite thing: the pronunciation.

Leaving the article out would usually sound less natural here. Greek often keeps the article with abstract nouns or general concepts more than English does.

So:

• η προφορά = natural here
• προφορά without the article would usually not be the best choice in this sentence

Not exactly.

προφορά usually means pronunciation: the way sounds are produced.

Depending on context, English accent can mean different things:

• a foreign or regional accent
• a written stress mark
• the way someone pronounces words

Greek uses different words depending on which meaning you want. In this sentence, προφορά clearly means pronunciation.

You often learn noun gender together with the article:

• η δυσκολία
• η προφορά

That is the safest habit in Greek: memorize nouns with their article.

Also, many nouns ending in -ία and -ά are feminine, though you should not rely on endings alone 100% of the time.