Κατεβαίνω τη σκάλα αργά γιατί πονάει το γόνατό μου.

Here κατεβαίνω means I go down, I come down, or I descend.

It is the 1st person singular present tense, so it matches I:

• κατεβαίνω = I go down / I am going down

In other contexts, κατεβαίνω can also mean things like get off or go down from somewhere, but in this sentence it clearly means going down the stairs.

Because the idea of downward movement is already built into the verb κατεβαίνω.

So:

• κατεβαίνω τη σκάλα = literally I descend the staircase
• natural English = I go down the stairs

English often uses a verb plus a particle or preposition, like go down. Greek often packs that meaning into one verb.

Greek often uses η σκάλα to mean the staircase or the stairs as a single unit.

So:

• κατεβαίνω τη σκάλα = I go down the stairs / staircase

Using the singular is very natural in Greek when you mean one stairway. A plural form can exist in other contexts, but τη σκάλα is completely normal here.

Also, τη σκάλα is in the accusative, because it is the direct object of κατεβαίνω in this sentence.

The full form of the feminine accusative singular article is την, but the final -ν is often dropped before certain consonants in normal modern Greek spelling and speech.

So:

• την σκάλα → commonly τη σκάλα

Before σ, dropping the -ν is very common and natural. So τη σκάλα is standard.

Because αργά is an adverb, and it describes how the action happens.

Here it tells us how I go down the stairs:

• αργά = slowly

By contrast:

• αργός = slow masculine adjective
• αργή = slow feminine adjective
• αργό = slow neuter adjective

So:

• αργός άνθρωπος = a slow person
• κατεβαίνω αργά = I go down slowly

It can mean both, depending on context.

In this sentence, γιατί means because, since it introduces the reason:

• ... γιατί πονάει το γόνατό μου = ... because my knee hurts

As a question word, γιατί can also mean why?

• Γιατί αργείς; = Why are you late?

So you understand it from the structure and punctuation:

• inside a statement: often because
• as a question word: why

πονάει is the 3rd person singular present of the verb πονάω / πονώ, meaning to hurt or to be in pain.

The subject is:

• το γόνατό μου = my knee

So Greek says:

• πονάει το γόνατό μου = my knee hurts

It is 3rd person singular because το γόνατό μου is singular.

You may also hear:

• πονάει
• πονά

Both are common modern forms.

Greek usually expresses possession with:

• definite article + noun + weak possessive pronoun

So:

• το γόνατό μου = literally the knee my
• natural English = my knee

This is very normal Greek, especially with body parts, clothing, and close personal things.

So Greek often prefers:

• πονάει το γόνατό μου = my knee hurts

rather than trying to mirror English word-for-word.

This happens because μου is a weak pronoun, often called an enclitic. When an enclitic follows a word like γόνατο, Greek often adds an extra written accent.

So:

• alone: γόνατο
• with μου: γόνατό μου

This does not mean it is a different word. It is the same noun, but the accent pattern changes in writing because of the following μου.

You will see the same thing in many similar expressions.

No. Greek word order is more flexible than English.

This sentence uses:

• πονάει το γόνατό μου

But you could also say:

• το γόνατό μου πονάει

Both mean the same thing: my knee hurts.

The version in your sentence is very natural in spoken Greek. Greek often moves words around for rhythm, emphasis, or style, while keeping the meaning clear through endings and articles.