Learn Hardcore Greek: Αφήνω τα κλειδιά μου στο τραπέζι όταν μπαίνω στο σπίτι. - I leave my keys on the table when I enter the house.

To leave something somewhere
- Αφήνω τα κλειδιά στο τραπέζι. – I leave the keys on the table.
To let / allow
- Μη με αφήνεις μόνο. – Don’t leave me alone / don’t let me be alone.
To stop / give up
- Άφησα το κάπνισμα. – I gave up smoking.

Here it clearly has meaning (1): putting the keys somewhere and not taking them with you.

Why is it τα κλειδιά and not something else? What are the gender and number here?

Κλειδιά is the plural form of κλειδί (key).

το κλειδί – the key (neuter, singular)
τα κλειδιά – the keys (neuter, plural)

In this sentence:

τα = definite article, neuter plural, accusative
κλειδιά = neuter plural, accusative (direct object of αφήνω)

So τα κλειδιά literally means the keys, grammatically: the (neuter plural) keys (neuter plural).

Why do we say τα κλειδιά μου (article + noun + my)? Why not just κλειδιά μου for my keys?

In Greek, it’s normal and very common to use:

article + noun + possessive clitic

So:

τα κλειδιά μου = my keys
το βιβλίο σου = your book
η τσάντα του = his bag

You can sometimes drop the article (κλειδιά μου), but:

With concrete, specific things (like my keys), the version with the article (τα κλειδιά μου) is the default natural way.
Without the article can sound a bit more poetic, emphatic, or marked, and is less common in everyday speech.

So for everyday Greek, think: almost always use the article with a possessed noun: τα κλειδιά μου.

Could I say μου τα κλειδιά instead of τα κλειδιά μου?

In this kind of sentence, μου τα κλειδιά is not natural modern Greek.

The normal pattern is:

article + noun + possessive clitic
- τα κλειδιά μου – my keys
- το σπίτι μας – our house

You do see μου before the noun in some fixed or very emotional/poetic expressions (and more often in older or dialectal Greek), for example:

της μάνας μου το σπίτι – my mother’s house
της καρδιάς μου κομμάτι – a piece of my heart

But for simple my keys, stick to τα κλειδιά μου.

Why is there no word for I? Why don’t we say Εγώ αφήνω τα κλειδιά μου…?

Greek is a pro‑drop language: the subject pronoun is usually omitted because the verb ending already shows the person.

Αφήνω clearly tells us I leave (1st person singular).
So εγώ is not needed unless you want to emphasize I.

Compare:

Αφήνω τα κλειδιά μου στο τραπέζι. – I leave my keys on the table.
Εγώ αφήνω τα κλειδιά μου στο τραπέζι. – I leave my keys on the table (implying contrast: I do it, maybe others don’t).

In neutral statements, you normally drop εγώ.

What tense/aspect is Αφήνω here? Why not a different tense?

Αφήνω is in the present tense, imperfective aspect, 1st person singular.

In Greek, the present imperfective can express:

an action happening now
a habitual / repeated action

Here it clearly describes a habit:

Αφήνω τα κλειδιά μου στο τραπέζι όταν μπαίνω στο σπίτι.
→ I usually/always leave my keys on the table when I enter the house.

If you wanted to talk about a single completed event in the past, you’d use the aorist:

Άφησα τα κλειδιά μου στο τραπέζι. – I left my keys on the table (once, completed).

What exactly is στο? Is it one word or two words combined?

Στο is a contraction of:

σε (preposition: in, at, on, to)
το (neuter singular article: the)

So:

σε + το = στο

In the sentence we have:

στο τραπέζι = on the table / at the table
στο σπίτι = in/into the house

The preposition σε is very flexible and is translated as in, at, on, to, into depending on context.

Why is it στο τραπέζι for on the table? Doesn’t σε usually mean in/at rather than on?

Greek σε is very broad and covers:

in
at
on
to / into (motion)

The exact English preposition depends on the noun and the verb.

Here:

Αφήνω κάτι στο τραπέζι – I leave something on the table.
Even though σε by itself is general, στο τραπέζι in this context corresponds to on the table.

If you want to be more explicit about on (on top of), you can say:

πάνω στο τραπέζι – on (top of) the table

So:

Αφήνω τα κλειδιά μου στο τραπέζι.
Αφήνω τα κλειδιά μου πάνω στο τραπέζι.

Both are correct. The second slightly emphasizes the on top of idea.

Why do we also have στο σπίτι? Does στο here mean into or in?

With μπαίνω, which means to enter / to go in, σε usually corresponds to English into or in.

μπαίνω στο σπίτι
Literally: I enter to the house → naturally: I enter the house / go into the house.

So:

Αφήνω τα κλειδιά μου στο τραπέζι όταν μπαίνω στο σπίτι.
→ I leave my keys on the table when I go into the house / when I enter the house.

Again, Greek reuses σε, and English picks in/into according to the verb.

What’s the difference between σπίτι, το σπίτι, and στο σπίτι?

σπίτι by itself can mean:
- a house as a type of building
- home in a general sense (without article)
το σπίτι means the house (a specific house, as an object or building).
στο σπίτι = σε + το σπίτι
→ in/at the house, at home.

Some nuances:

Είμαι σπίτι. – I am (at) home.
(Here σπίτι functions like an adverb of place, no article.)
Είμαι στο σπίτι. – I am at the house / at home.
Very similar meaning in everyday speech.

In your sentence, στο σπίτι focuses on the physical place you are entering: into the house / into the home.

Could we say μπαίνω στο σπίτι μου instead of μπαίνω στο σπίτι? What changes?

Yes:

μπαίνω στο σπίτι – I enter the house / I go into the house
μπαίνω στο σπίτι μου – I enter my house / I go into my home

In many everyday contexts, στο σπίτι will be understood as my house / our house from context, so adding μου is often not necessary unless:

you want to emphasize it’s your house (not someone else’s)
the context is ambiguous.

So your sentence could also be:

Αφήνω τα κλειδιά μου στο τραπέζι όταν μπαίνω στο σπίτι μου.

Perfectly correct, just a bit more explicit.

Why is it όταν μπαίνω with the present, not something like a future form (when I will enter)?

In Greek, after όταν (when) introducing a general / habitual time clause, you normally use the present tense, imperfective aspect, even if English might use present simple or sometimes present with future sense.

Here:

όταν μπαίνω στο σπίτι
literally: when I enter the house
function: whenever / every time I enter the house

So:

Αφήνω τα κλειδιά μου στο τραπέζι όταν μπαίνω στο σπίτι.
→ Whenever I enter the house, I leave my keys on the table.

If you were talking about a specific future event, you’d use a future form:

Όταν θα μπω στο σπίτι, θα σε πάρω τηλέφωνο.
– When I (will) enter the house, I’ll call you.
(Even here, many speakers simply say Όταν μπω…; this is a different construction with the subjunctive.)

What is the difference between όταν and αν here? Could I say Αφήνω τα κλειδιά μου στο τραπέζι αν μπαίνω στο σπίτι?

In this sentence, you need όταν, not αν.

όταν = when (time)
→ introduces a time clause: when(ever) I enter the house
αν = if (condition)
→ introduces a conditional clause: if I enter the house

Your sentence describes a regular time‑based habit:

Every time (when) I enter the house, I leave my keys on the table.

Using αν μπαίνω στο σπίτι would sound wrong / unnatural here, because it would suggest a conditional situation (if I happen to be entering the house…), which doesn’t fit this habitual pattern.

How flexible is the word order? Could I say Αφήνω στο τραπέζι τα κλειδιά μου όταν μπαίνω στο σπίτι?

Yes, Greek word order is quite flexible. These are all acceptable:

Αφήνω τα κλειδιά μου στο τραπέζι όταν μπαίνω στο σπίτι.
Αφήνω στο τραπέζι τα κλειδιά μου όταν μπαίνω στο σπίτι.

Both are natural. The first is probably the most neutral. Changing the order can slightly shift emphasis:

Αφήνω στο τραπέζι τα κλειδιά μου…
puts a bit more focus on στο τραπέζι (on the table) as the place where you leave them.

But grammatically, both are fine; the roles of the words are shown mainly by articles and endings, not just position.