Το ελικόπτερο πετάει χαμηλά.

Το means the.

It is the definite article used with neuter singular nouns in Greek. Since ελικόπτερο is a neuter noun, it takes το in the nominative singular:

• το ελικόπτερο = the helicopter

Greek nouns have grammatical gender, and ελικόπτερο is neuter.

The basic singular definite articles are:

• ο for masculine
• η for feminine
• το for neuter

So:

• το ελικόπτερο = the helicopter

This does not mean a helicopter is biologically “neuter”; it is just the noun class it belongs to in Greek.

Πετάει is the 3rd person singular present form of the verb πετάω / πετώ, meaning to fly.

So πετάει can mean:

• flies
• is flying

depending on context.

In this sentence:

• Το ελικόπτερο πετάει χαμηλά.
• The helicopter flies low.
• or The helicopter is flying low.

Greek present tense often covers both the simple present and the present continuous that English separates.

Because Greek does not usually make the same tense distinction that English does here.

In English, you often choose between:

• The helicopter flies low = general/habitual
• The helicopter is flying low = happening right now

In Greek, the present tense πετάει can express either one, and context tells you which meaning is intended.

So this sentence could describe:

• a general characteristic, or
• an action happening at the moment

Χαμηλά here is an adverb, meaning low or at a low altitude.

It modifies the verb πετάει:

• πετάει χαμηλά = flies low / is flying low

It is not an adjective in this sentence, so it does not agree with ελικόπτερο.

Compare:

• Το ελικόπτερο είναι χαμηλό. = The helicopter is low.
  Here χαμηλό is an adjective describing the noun.

• Το ελικόπτερο πετάει χαμηλά. = The helicopter flies low.
  Here χαμηλά is an adverb describing how it flies.

Very often, yes, but forms like this can sometimes overlap in function depending on context.

In this sentence, it is clearly adverbial because it describes the action:

• πετάει χαμηλά = flies low

A learner should mainly remember this practical distinction:

• χαμηλός / χαμηλή / χαμηλό = adjective forms (low)
• χαμηλά = adverb (low, at a low level, down low)

In normal neutral Greek, το is expected here if you mean the helicopter.

• Το ελικόπτερο πετάει χαμηλά. = The helicopter is flying low.

If you remove the article, the result sounds less natural in an ordinary standalone sentence. Greek uses the definite article more regularly than English in many situations.

If you meant a helicopter, you would often simply say:

• Ένα ελικόπτερο πετάει χαμηλά. = A helicopter is flying low.

The given order is the most neutral:

• Το ελικόπτερο πετάει χαμηλά.

But Greek word order is fairly flexible. You could change it for emphasis:

• Χαμηλά πετάει το ελικόπτερο. = The helicopter is flying low.
  (emphasis on low)

• Πετάει χαμηλά το ελικόπτερο.
  Also possible, depending on context.

So the basic meaning stays the same, but the emphasis or tone may change.

Πετάει is pronounced roughly pe-TA-i.

The accent mark shows which syllable is stressed:

• πε-ΤΑ-ει

Stress matters in Greek pronunciation, and learners should pay attention to it from the beginning.

A rough pronunciation of the whole sentence is:

• to e-li-KO-pte-ro pe-TA-i ha-mi-LA

This is only an approximation, but it helps with stress placement.

It is in the nominative singular because it is the subject of the sentence.

• το ελικόπτερο = the helicopter
• it is the thing doing the action of πετάει

So:

• Το ελικόπτερο = subject
• πετάει = verb
• χαμηλά = adverb

The dictionary form is ελικόπτερο.

It is a neuter noun, and like other Greek nouns, it changes form depending on case and number. For example:

• το ελικόπτερο = nominative singular
• του ελικοπτέρου = genitive singular
• τα ελικόπτερα = nominative/accusative plural

A beginner mainly needs to recognize here that το ελικόπτερο is the normal subject form: the helicopter.

Yes. This verb is commonly seen in more than one present-tense pattern.

You may encounter forms connected with:

• πετάω
• πετώ

So learners may see things like:

• πετάω
• πετάς
• πετάει

and also forms based on πετώ in some contexts or styles.

For everyday learning, the important thing in this sentence is simply:

• πετάει = he/she/it flies or is flying

In normal use, it is most naturally literal:

• The helicopter is flying low.

Nothing in the sentence itself strongly suggests a figurative meaning. It is a straightforward example of:

• subject + verb + adverb

So for a learner, the main value of the sentence is grammatical:

• το = article
• ελικόπτερο = noun
• πετάει = present-tense verb
• χαμηλά = adverb