Όταν είμαι κουρασμένος, ακουμπάω το κεφάλι μου στο μαξιλάρι και χαλαρώνω.

Greek often leaves out subject pronouns because the verb ending already shows who the subject is.

• είμαι = I am
• ακουμπάω = I rest / I put / I lean
• χαλαρώνω = I relax

So εγώ is not necessary. You can add εγώ for emphasis or contrast, but the normal neutral sentence usually omits it.

Όταν means when or whenever.

In this sentence, it introduces a time clause:

• Όταν είμαι κουρασμένος = When/Whenever I am tired

The comma is used because this time clause comes first, before the main clause. Greek punctuation here works much like English:

• When I am tired, ...

If the main clause came first, the comma would often disappear:

• Ακουμπάω το κεφάλι μου στο μαξιλάρι όταν είμαι κουρασμένος.

Κουρασμένος is an adjective meaning tired, and it agrees with the person speaking.

Here it is masculine singular, so it suggests the speaker is male.

• masculine: κουρασμένος
• feminine: κουρασμένη
• masculine plural: κουρασμένοι
• feminine plural: κουρασμένες

So a female speaker would say:

• Όταν είμαι κουρασμένη, ...

The verb είμαι stays the same. Only the adjective changes.

It is commonly used like an adjective, but historically it comes from a participial form. For learners, the most useful thing is to treat it as an adjective that goes with είμαι:

• είμαι κουρασμένος = I am tired
• είμαι χαρούμενος = I am happy
• είμαι έτοιμος = I am ready

So in practice, you can learn είμαι + adjective as a very common Greek pattern.

Yes. Ακουμπάω and ακουμπώ are both common Modern Greek forms of the same verb.

• ακουμπάω = full form
• ακουμπώ = contracted form

Both are natural. In this sentence, the verb means something like:

• rest
• place
• lean

So ακουμπάω το κεφάλι μου στο μαξιλάρι means I rest/put my head on the pillow.

In Greek, possessive words like μου usually come after the noun:

• το κεφάλι μου = my head
• το βιβλίο μου = my book
• η φίλη μου = my friend

This is the normal pattern:

• article + noun + μου/σου/του...

So Greek literally looks more like the head my than my head.

Because Greek normally keeps the definite article with possessed nouns.

So Greek says:

• το κεφάλι μου = literally the head my
• το σπίτι μου = literally the house my

This is completely normal Greek. The article is not extra or unusual here.

Greek also very often uses the article with body parts, so το κεφάλι is exactly what you would expect.

Στο is a contraction of:

• σε + το = στο

The preposition σε can mean in, at, to, or on, depending on the context.

So:

• στο μαξιλάρι = on the pillow / onto the pillow

This is very common in Greek:

• στο σπίτι = at home / to the house
• στο τραπέζι = on the table
• στο σχολείο = at/to school

Because Greek σε covers several location ideas that English splits into different words such as in, at, to, and sometimes on.

So Greek does not always match English prepositions one-to-one. Here, σε plus the article gives:

• στο μαξιλάρι

Even though the most natural English translation is on the pillow, Greek still uses σε.

This is something learners have to get used to: Greek prepositions often work a bit more broadly than English ones.

Because this sentence describes a usual or repeated action, not one single event.

So the idea is:

• When I am tired, I put my head on the pillow and relax.
• more naturally: Whenever I am tired, this is what I do.

Greek uses the present tense for this kind of habitual meaning, just as English often does.

If you were talking about one past event, you would use past forms instead.

It can mean either, depending on context. In a sentence like this, it usually has a habitual sense, so whenever I am tired is often the best way to understand it.

That is because the rest of the sentence also describes a repeated action:

• I rest my head on the pillow and relax

So even though όταν is often translated as when, the overall meaning here is probably more general than a single occasion.

Its basic meaning is and.

In this sentence, it connects two actions:

• ακουμπάω το κεφάλι μου στο μαξιλάρι
• και χαλαρώνω

Because of the real-world logic, it naturally sounds like a sequence: first the head goes on the pillow, then relaxation happens. But Greek does not need a special word for then here. Context makes the order clear.

Because both nouns are neuter singular:

• το κεφάλι = the head
• το μαξιλάρι = the pillow

A very common clue is the ending -ι. Many Greek nouns ending in -ι are neuter and take το.

Still, it is best to learn nouns together with their article:

• το κεφάλι
• το μαξιλάρι

That helps you remember grammatical gender correctly from the start.

Only the adjective would need to change:

• masculine: Όταν είμαι κουρασμένος, ακουμπάω το κεφάλι μου στο μαξιλάρι και χαλαρώνω.
• feminine: Όταν είμαι κουρασμένη, ακουμπάω το κεφάλι μου στο μαξιλάρι και χαλαρώνω.

Everything else stays the same.

That is because the verbs are first person singular either way, but the adjective must agree with the speaker's gender.