Ο φωτογραφικός φακός είναι καθαρός.

ο is the Greek definite article meaning the for a masculine singular noun in the nominative case.

So:

• ο φακός = the lens

Greek usually uses the article in places where English also uses the, and sometimes even a bit more often.

φωτογραφικός is an adjective. It means photographic or camera-related.

It describes φακός, so:

• φωτογραφικός φακός = photographic lens / camera lens

Grammatically, it matches φακός in:

• gender: masculine
• number: singular
• case: nominative

That is why it ends in -ός.

No. φακός can mean different things depending on context, including:

• lens
• flashlight / torch

In this sentence, because of φωτογραφικός, it clearly means lens, not flashlight.

So ο φωτογραφικός φακός is specifically a photographic lens.

είναι means is.

It is the 3rd person singular present form of the verb είμαι = to be.

So:

• είμαι = I am
• είναι = he/she/it is

In this sentence, είναι links the subject with the adjective:

• Ο φωτογραφικός φακός είναι καθαρός.
• The camera lens is clean.

Greek does not need a separate subject pronoun like it here, because the subject is already stated:

• Ο φωτογραφικός φακός

So Greek simply says:

• The photographic lens is clean

There is no need to add a separate word meaning it.

Because καθαρός has to agree with φακός.

φακός is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives:

• masculine: καθαρός
• feminine: καθαρή
• neuter: καθαρό

Compare:

• ο φακός είναι καθαρός = the lens is clean
• η κάμερα είναι καθαρή = the camera is clean
• το τζάμι είναι καθαρό = the glass is clean

Because καθαρός is a predicate adjective, not part of the noun phrase.

In other words:

• ο φωτογραφικός φακός = the subject
• είναι καθαρός = tells us what the subject is like

In Greek, predicate adjectives normally appear without an article after verbs like είναι.

So:

• ο φακός είναι καθαρός = correct

Adding an article before καθαρός would change the structure and usually would not fit this simple sentence.

The main noun phrase is in the nominative case, because it is the subject of the sentence:

• ο
• φωτογραφικός
• φακός

The adjective καθαρός is also nominative because it agrees with the subject as a predicate adjective.

So everything describing the subject is in the nominative here.

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

The most neutral order here is:

• Ο φωτογραφικός φακός είναι καθαρός.

But you may also hear:

• Καθαρός είναι ο φωτογραφικός φακός.
• Ο φακός είναι καθαρός.

Changing the order usually changes the focus or emphasis, not the basic meaning.

For a learner, the original order is the safest and most natural starting point.

A rough pronunciation is:

• o fo-to-gra-fi-KOS fa-KOS EE-ne ka-tha-ROS

A few helpful notes:

• The stressed syllables are marked by the accent marks:
  φωτογραφικός, φακός, καθαρός
• αι in είναι is pronounced like e in modern Greek.
• θ in καθαρός sounds like th in think.

So είναι sounds roughly like EE-ne.

Not necessarily. Greek can express this idea in more than one way.

For example, depending on context, you might also hear something like:

• ο φακός της κάμερας = the lens of the camera

But ο φωτογραφικός φακός is a correct and natural phrase meaning photographic lens / camera lens.

It sounds a bit more descriptive, while ο φακός της κάμερας sounds more explicitly like the lens belonging to the camera.