Η ακτίνα είναι χαλαρή.

Η is the definite article, meaning the.
Here it is the feminine singular nominative form, because ακτίνα is a feminine singular noun and it is the subject of the sentence.

A quick article set is:

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

So Η ακτίνα = the ακτίνα.

You can tell from the words agreeing with it:

• η is feminine singular
• χαλαρή is also feminine singular

Greek adjectives and articles must agree with the noun in gender, number, and case. So even if you did not already know the gender of ακτίνα, the sentence shows it.

Also, many nouns ending in -α are feminine, and ακτίνα is one of them.

Because Greek adjectives change form to match the noun they describe.

The basic forms are:

• χαλαρός = masculine
• χαλαρή = feminine
• χαλαρό = neuter

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

• Η ακτίνα είναι χαλαρή.

If the noun were neuter, you would use χαλαρό. If it were masculine, you would use χαλαρός.

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

In this sentence, it means is.

A useful point: είναι is the same form for both:

• he/she/it is
• they are

So Greek does not change this form the way English changes is and are.

It is in the nominative case, because it is the subject of the sentence.

The structure is:

• Η ακτίνα = subject
• είναι = verb
• χαλαρή = predicate adjective

The adjective χαλαρή is also in the nominative, because it describes the subject after the verb to be.

A rough pronunciation is:

ee ak-TEE-na EE-ne ha-la-REE

More carefully:

• Η = ee
• ακτίνα = ak-TEE-na
• είναι = EE-ne
• χαλαρή = ha-la-REE or a more throaty sound at the start than English h

The letter χ in χαλαρή is not exactly the English h. It is a Greek sound that can sound like the ch in Scottish loch or German Bach, though many learners start with a simple h approximation.

The accent marks show which syllable gets the stress.

• ακτίνα → stress on τί
• είναι → stress on εί
• χαλαρή → stress on the last syllable, ρή

The article η has no accent because it is unstressed.

Stress matters in Greek, so the accent marks are a normal and important part of spelling.

Greek word order is more flexible than English word order, but the sentence as written is the neutral, straightforward order:

• Η ακτίνα είναι χαλαρή.

You may also see different orders for emphasis, for example:

• Χαλαρή είναι η ακτίνα.

That puts more focus on χαλαρή.

For a learner, the safest basic pattern is:

article + noun + είναι + adjective

Usually, no—not if you mean the in a normal sentence.

Greek uses the definite article very naturally and very often. With a specific noun like this, η is normally expected:

• Η ακτίνα είναι χαλαρή.

Without the article, the sentence sounds unnatural in ordinary usage unless there is some very special context.

Grammatically, yes.

Whether it sounds natural in everyday life depends on what ακτίνα means in the context. A learner may know one meaning, but ακτίνα can refer to different things depending on context. With some meanings, χαλαρή can sound perfectly normal; with others, it may sound unusual or technical.

So the grammar is very standard, even if the full sentence may feel more or less natural depending on the situation.