Ο σκελετός είναι ελαφρύς.

Ο means the. It is the masculine singular nominative form of the definite article.

It is used here because σκελετός is:

• masculine
• singular
• the subject of the sentence

So ο σκελετός = the skeleton.

In Greek, grammatical gender is not the same thing as biological sex.

Σκελετός is grammatically masculine, so it takes:

• the masculine article ο
• masculine adjective forms such as ελαφρύς

This does not mean the skeleton is male. It is just how the noun behaves grammatically.

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

In this sentence, it means is:

• Ο σκελετός είναι ελαφρύς. = The skeleton is light.

A useful thing to know is that είναι can also mean are, depending on the subject:

• Ο σκελετός είναι... = The skeleton is...
• Οι σκελετοί είναι... = The skeletons are...

Greek usually does not need a subject pronoun when the subject is already stated.

English says:

• The skeleton is light.

Greek also just says:

• Ο σκελετός είναι ελαφρύς.

You do not need a separate word for it because ο σκελετός already tells you what the subject is.

Because it has to agree with σκελετός.

In Greek, adjectives change form to match the noun in:

• gender
• number
• case

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

• ελαφρύς = masculine singular

So:

• ο σκελετός είναι ελαφρύς

It would change to match the noun.

Common singular forms are:

• ελαφρύς = masculine
• ελαφριά = feminine
• ελαφρύ = neuter

Examples:

• Η τσάντα είναι ελαφριά. = The bag is light.
• Το κουτί είναι ελαφρύ. = The box is light.

So the adjective must match the noun’s gender.

Here ελαφρύς is a predicate adjective, not a directly attached adjective inside the noun phrase.

The structure is:

• subject + είναι + adjective

So:

• Ο σκελετός = the subject
• είναι = is
• ελαφρύς = light

This is like English The skeleton is light.

If you wanted an adjective directly attached to the noun, that would be a different structure, for example:

• ο ελαφρύς σκελετός = the light skeleton

Yes, Greek word order is more flexible than English.

The most neutral order here is:

• Ο σκελετός είναι ελαφρύς.

But other orders are possible for emphasis, such as:

• Ελαφρύς είναι ο σκελετός.

That sounds more marked or emphatic, something like:

• It’s the skeleton that is light or
• The skeleton is light, not heavy

For learners, the given order is the safest and most natural one to use first.

The accent mark shows which syllable is stressed in pronunciation.

So these are pronounced roughly as:

• σκελετός
• είναι
• ελαφρύς

Stress matters in Greek, so you should learn words with their accents.

A rough pronunciation guide:

• Ο σκελετός είναι ελαφρύς
  o skele-TOS EE-ne elaf-REES

Greek has two written forms of lowercase sigma:

• σ in the middle of a word
• ς at the end of a word

So in this sentence:

• σκελετός ends with ς
• ελαφρύς also ends with ς

They are the same letter, just written differently depending on position.

In normal Greek, that would usually sound unnatural here.

If you mean the skeleton, you normally keep the article:

• Ο σκελετός είναι ελαφρύς.

If you mean a skeleton, you could say:

• Ένας σκελετός είναι ελαφρύς.

Greek uses articles a lot, often more consistently than English learners expect.

Here it means light in weight.

Greek usually uses different words for other meanings of English light:

• light = not heavy → ελαφρύς
• light = bright / not dark in color → usually something else, depending on context

So in this sentence, the meaning is definitely about weight.

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

You can tell from:

• the article ο, which is masculine singular nominative
• the sentence role: the skeleton is the thing being described

So:

• ο σκελετός = nominative subject
• ελαφρύς matches it in the nominative as well

This is why the whole sentence is grammatically consistent.