Ο στίχος είναι όμορφος.

Ο is the masculine singular form of the definite article, so here it means the.

It matches στίχος, which is a masculine singular noun. Greek articles change form depending on gender, number, and case.

Because στίχος is a masculine singular noun in the nominative case here, and the matching definite article is ο.

Greek articles must agree with the noun, so you cannot just use any form of the.

Because όμορφος is an adjective, and in Greek adjectives must agree with the noun they describe.

Since στίχος is masculine, singular, and nominative, the adjective also appears in the masculine singular nominative form:

• ο στίχος είναι όμορφος = masculine
• η μελωδία είναι όμορφη = feminine
• το τραγούδι είναι όμορφο = neuter

So the -ος ending here shows agreement, not just dictionary form.

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

In this sentence it means is:

• Ο στίχος είναι όμορφος = The verse/line is beautiful

A useful thing to remember is that είναι is used for both:

• he/she/it is
• they are

So Greek does not distinguish those two in this form.

The accent marks show which syllable is stressed.

• στίχος → stress on στί-
• είναι → stress on εί-
• όμορφος → stress on ό-

Stress matters in Greek pronunciation, so the marks are important.

A rough pronunciation is:

o STÍ-hos Í-ne Ó-mor-fos

A more accurate phonetic guide is:

[o ˈstixos ˈine ˈomorfos]

A few notes:

• χ in στίχος is not like English h. It is a throat sound, like the ch in Scottish loch or German Bach.
• ει in είναι is pronounced like ee in see.

Greek has two lowercase forms of sigma:

• σ is used at the beginning or in the middle of a word
• ς is used only at the end of a word

So in στίχος, the first sigma is σ and the final sigma is ς. They are the same letter.

This is the most neutral word order: subject + verb + adjective.

Greek word order is more flexible than English, so you could also say:

• Όμορφος είναι ο στίχος

That would sound more emphatic, something like It is the verse that is beautiful or The verse is beautiful with extra focus on beautiful.

So the original sentence is the plain, standard order.

If you mean the verse/the line, then yes, the article is normally used.

Greek uses the definite article very regularly. English sometimes leaves articles out where Greek would not.

Without ο, the sentence would usually sound unnatural in ordinary speech unless there were some special context.

It can definitely describe things too.

Όμορφος can mean beautiful, lovely, nice-looking, and it is very natural with objects, places, music, language, writing, and so on.

So Ο στίχος είναι όμορφος is perfectly natural Greek.