Το ρεφρέν είναι εύκολο.

• Το = the definite article, neuter singular
• ρεφρέν = chorus / refrain
• είναι = is
• εύκολο = easy, in the neuter singular form

So grammatically, it is:

[subject] Το ρεφρέν + [verb] είναι + [predicate adjective] εύκολο

Το means the. Greek uses the definite article very regularly, often more than English does.

So:

• Το ρεφρέν = the chorus / the refrain
• Ρεφρέν by itself can appear in special contexts, but in a normal sentence like this, το is the natural choice.

Because ρεφρέν is a neuter noun in Greek. Grammatical gender in Greek does not always match any real-world idea of natural gender. Many inanimate nouns are masculine, feminine, or neuter simply because that is how the language classifies them.

A useful thing to notice is that ρεφρέν is a loanword, and words like this are often treated as neuter in Greek.

That is why you get:

• το ρεφρέν
• εύκολο to match it

Because adjectives must agree with the noun they describe.

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

• masculine: εύκολος
• feminine: εύκολη
• neuter: εύκολο

So:

• Το ρεφρέν είναι εύκολο. = correct

Compare:

• Ο στίχος είναι εύκολος. = The lyric/verse is easy.
• Η μελωδία είναι εύκολη. = The melody is easy.

Είναι is the present-tense form of είμαι, meaning to be.

Here it means is.

A detail learners often notice: είναι is used for both:

• he/she/it is
• they are

So:

• Το ρεφρέν είναι εύκολο. = The chorus is easy.
• Τα ρεφρέν είναι εύκολα. = The choruses are easy.

The verb stays είναι in both sentences.

A simple guide is:

To refrén íne éfkolo

A few key pronunciation points:

• ρεφρέν is stressed on the last syllable: re-FREN
• είναι is usually pronounced roughly EE-ne or I-ne, with the stress on the first part
• In εύκολο, the ευ sounds like ef here, because it comes before κ, a voiceless consonant

So εύκολο sounds like EF-ko-lo.

Because here εύκολο is not directly attached to the noun inside the noun phrase. It is a predicate adjective after είναι.

So:

• Το ρεφρέν είναι εύκολο = The chorus is easy.

If you put the adjective directly with the noun, you get a different structure:

• το εύκολο ρεφρέν = the easy chorus

That is not a full sentence by itself; it is just a noun phrase.

No. Greek word order is more flexible than English word order.

The neutral, straightforward order is:

• Το ρεφρέν είναι εύκολο.

But you may also hear:

• Είναι εύκολο το ρεφρέν.
• Εύκολο είναι το ρεφρέν.

These can shift the emphasis slightly, but the basic meaning stays the same. For a learner, the original order is the safest and most neutral one.

No, not in this sentence.

English often uses pronouns like it, but Greek does not need a separate dummy subject here. The actual subject is already present:

• Το ρεφρέν = the subject
• είναι = is
• εύκολο = easy

So Greek simply says:

• Το ρεφρέν είναι εύκολο.

Το ρεφρέν is in the nominative, because it is the subject of the sentence.

εύκολο is also in the nominative, because after είναι Greek normally uses a predicate adjective that agrees with the subject in gender, number, and case.

So both are:

• neuter
• singular
• nominative

That agreement is why the sentence looks so neat and regular:

• Το ρεφρέν είναι εύκολο.