Σήμερα με πονάει η πλάτη μου και το γόνατό μου, γιατί περπάτησα πολύ χτες.

They do different jobs.

• με = me. It is the object pronoun: it hurts me.
• μου = my / of me. It shows possession: my back, my knee.

So με πονάει η πλάτη μου is literally something like my back hurts me.

This is very common in Greek with body parts. English usually just says my back hurts, but Greek often makes the affected person explicit with με as well.

This verb can be used in more than one way.

1. The person in pain can be the subject

   • Πονάω. = I’m hurting / I’m in pain.

2. The body part can be the subject

   • Με πονάει η πλάτη μου. = literally My back hurts me.

In your sentence, the body parts are what hurt, so η πλάτη μου and το γόνατό μου function as the subject, and με is the person affected.

This is a very natural feature of colloquial Greek.

When the verb comes before a compound subject, Greek often uses the singular, especially in everyday speech:

• Με πονάει η πλάτη μου και το γόνατό μου.

You may also hear or see the plural:

• Με πονάνε η πλάτη μου και το γόνατό μου.

That plural agrees more strictly with the two subjects. But the singular in your sentence is very common and sounds natural in speech.

Greek normally uses the definite article with possessed nouns:

• η πλάτη μου = my back
• το γόνατό μου = my knee

So where English says just my back, Greek usually says literally the back my.

This is standard Greek, not extra emphasis.

Because each noun is being marked as possessed.

• η πλάτη μου = my back
• το γόνατό μου = my knee

If you said only η πλάτη μου και το γόνατο, it could sound as if only the first noun is clearly marked as mine, while the second is not.

Repeating μου is the clearest and most natural way to say that both body parts are yours.

This is due to a standard Greek accent rule with enclitics like μου.

The noun γόνατο is stressed on the third syllable from the end. When an enclitic such as μου follows it, Greek often adds an extra written accent:

• γόνατο
• γόνατό μου

This does not mean it is a different word. It is the same noun, just written according to the accent rules when followed by μου.

Περπάτησα is the aorist form of περπατάω / περπατώ.

It is used because the speaker is referring to a completed action in the past:

• χτες = yesterday
• περπάτησα πολύ = I walked a lot

The idea is: the walking happened yesterday as a completed event, and today the result is pain.

Because the sentence is talking about what happened, not emphasizing the ongoing process.

• περπάτησα = I walked / I did a lot of walking
  Focus: the action as a whole, completed.

• περπατούσα = I was walking / I used to walk
  Focus: duration, background, or repeated/habitual action.

Here the speaker is giving a simple reason for today’s pain, so περπάτησα πολύ χτες is the natural choice.

Because Greek usually drops subject pronouns when the verb ending already shows who the subject is.

• περπάτησα already means I walked

The ending -σα tells you it is first person singular.

You can add εγώ only if you want emphasis or contrast:

• Εγώ περπάτησα πολύ χτες. = I walked a lot yesterday.

But in a neutral sentence, Greek normally leaves εγώ out.

Because here πολύ is an adverb, modifying the verb περπάτησα.

• περπάτησα πολύ = I walked a lot

Compare:

• πολύ = adverb, a lot / very
• πολλά = adjective/pronoun, many / a lot of things (neuter plural)

So in this sentence, the standard form is πολύ because it describes how much the person walked.

Γιατί can mean either:

• why? in a question
• because in a statement

Here it introduces the reason, so it means because:

• ..., γιατί περπάτησα πολύ χτες.
• ..., because I walked a lot yesterday.

You know it means because from the sentence structure: it is connecting two clauses, not asking a question.

Both mean yesterday.

• χτες = very common modern spelling in everyday use
• χθες = more traditional spelling, also common

In pronunciation, they are essentially the same in Modern Greek. So this is mostly a spelling/register issue, not a meaning difference.

No. Greek word order is fairly flexible.

Your sentence starts with Σήμερα to set the time frame right away:

• Σήμερα με πονάει η πλάτη μου και το γόνατό μου...

That is very natural. Greek often moves words around for focus, topic, or rhythm.

For example, you could also hear:

• Με πονάει η πλάτη μου σήμερα...
• Χτες περπάτησα πολύ, γι’ αυτό σήμερα με πονάει...

The version you have is natural and clear, but it is not the only possible order.

Yes. In everyday Modern Greek, both forms are used for the 3rd person singular of πονάω:

• πονάει
• πονά

So these are both possible:

• Με πονάει η πλάτη μου.
• Με πονά η πλάτη μου.

They mean the same thing. Πονάει is often felt as slightly fuller in form, but both are standard and common.

Because the speaker is talking first about the current situation:

• Σήμερα = what is true today

Then the sentence gives the cause:

• γιατί περπάτησα πολύ χτες = because I walked a lot yesterday

So the timeline is:

1. Today: my back and knee hurt.
2. Yesterday: I walked a lot.
3. Yesterday’s action explains today’s pain.

That ordering is very natural in Greek, just as it is in English.