Η αλυσίδα είναι καθαρή.

Η is the feminine singular definite article in Greek, equivalent to the in English.

So:

• η αλυσίδα = the chain

Greek usually uses the definite article more regularly than English does, and here it is the normal way to say the chain.

Also note that Η is capitalized only because it is the first word of the sentence. In lowercase, it is η.

In Greek, every noun has a grammatical gender: masculine, feminine, or neuter.
Αλυσίδα is a feminine noun, so it takes:

• the feminine article: η
• the feminine form of the adjective: καθαρή

This is grammatical gender, not biological gender. A thing like a chain can still be grammatically feminine.

Because adjectives in Greek must agree with the noun they describe in:

• gender
• number
• case

Here, αλυσίδα is:

• feminine
• singular
• nominative

So the adjective must match it:

• καθαρός = masculine singular
• καθαρή = feminine singular
• καθαρό = neuter singular

So:

• η αλυσίδα είναι καθαρή = the chain is clean

Είναι is the 3rd person singular form of the verb είμαι (to be).

Here it means is.

So the structure is:

• Η αλυσίδα = the chain
• είναι = is
• καθαρή = clean

This is a very common Greek sentence pattern:

noun + είναι + adjective

A natural pronunciation is:

• Η → i
• αλυσίδα → a-li-SI-tha
• είναι → I-ne
• καθαρή → ka-tha-RI

More approximately in IPA:

[i aliˈsiða ˈine kaθaˈri]

A few helpful sound notes:

• θ sounds like th in think
• δ sounds like th in this
• η, ι, υ, ει, οι are all pronounced like ee in Modern Greek

Because Greek does not need a subject pronoun here.

In English, we often say:

• It is clean

But in Greek, if the subject noun is already there, you just say:

• Η αλυσίδα είναι καθαρή

Greek normally does not add an extra pronoun unless there is special emphasis.

Yes, Greek word order is often more flexible than English word order.

The most neutral order here is:

• Η αλυσίδα είναι καθαρή.

But you might also hear:

• Καθαρή είναι η αλυσίδα.

That alternative gives more emphasis to clean or creates contrast, something like:

• It’s the chain that is clean
• The chain is clean

For a learner, the original order is the safest and most natural pattern to use.

Usually here, yes: it means clean in the ordinary physical sense.

But like English clean, Greek καθαρός / καθαρή / καθαρό can also have broader meanings depending on context, such as:

• pure
• clear
• neat
• spotless

In this sentence, with αλυσίδα, the most natural reading is simply the chain is clean.

Greek uses accent marks to show which syllable is stressed.

• καθαρή has the stress on the last syllable: ka-tha-RI
• αλυσίδα has the stress on σί: a-li-SI-tha
• είναι has the stress on the first syllable: EI-ne

The article η is a very short unstressed word, so it normally has no accent.

You add δεν before the verb:

• Η αλυσίδα δεν είναι καθαρή.
• The chain is not clean.

This is the normal way to negate είναι.

You would use the feminine singular indefinite article:

• Μια αλυσίδα είναι καθαρή.

That means a chain is clean.

However, in real usage, this sentence may sound a bit context-dependent, because English and Greek often prefer a more specific context for something like that. Still, grammatically it is correct.

Compare:

• Η αλυσίδα είναι καθαρή. = The chain is clean.
• Μια αλυσίδα είναι καθαρή. = A chain is clean.

It is in the nominative singular.

That is because αλυσίδα is the subject of the sentence:

• The chain is clean.

The article and adjective reflect that same grammar:

• η = feminine singular nominative article
• αλυσίδα = feminine singular nominative noun
• καθαρή = feminine singular nominative adjective

The pattern stays the same, but the article and adjective change to match the noun.

Examples:

• Ο κρίκος είναι καθαρός. = The link/ring is clean.
  Masculine singular

• Το μέταλλο είναι καθαρό. = The metal is clean.
  Neuter singular

So the adjective changes like this:

• καθαρός = masculine
• καθαρή = feminine
• καθαρό = neuter

That agreement system is one of the key things to notice in Greek.