Το κλειδί μου είναι κάτω από την καρέκλα.

Yes, my key is already definite in English, but Greek normally still uses the definite article with possessives.

So:

• το κλειδί μου = my key
• literally: the key my

This is very common in Greek. So even though English says my key, Greek usually says the key my.

In Greek, unstressed possessive forms like μου (my) usually come after the noun.

So:

• το κλειδί μου = my key
• η τσάντα μου = my bag
• το σπίτι μου = my house

This is the normal pattern. English puts the possessive before the noun, but Greek usually puts this short form after it.

μου is the weak possessive form meaning my.

It can also mean to me or for me in other contexts, but here, after a noun, it means possession:

• το κλειδί μου = my key

If you want to be more emphatic, Greek can also use:

• το δικό μου κλειδί = my key / my own key

But in your sentence, simple το κλειδί μου is the natural everyday form.

In this sentence, Greek uses the verb είναι = is.

So:

• Το κλειδί μου είναι κάτω από την καρέκλα.
• My key is under the chair.

Unlike some languages, standard Greek normally does not omit is in this kind of sentence. You usually need είναι.

Yes, κάτω από works together as under / below.

The parts are:

• κάτω = down / below
• από = from / by / than / under depending on context

Together, κάτω από means:

• under
• below

Examples:

• κάτω από το τραπέζι = under the table
• κάτω από την καρέκλα = under the chair

So learners should usually treat κάτω από as a set expression.

Because after από, the noun phrase goes in the accusative case.

The dictionary form is:

• η καρέκλα = the chair (nominative)

But after από, it becomes:

• την καρέκλα = the chair (accusative)

So:

• η καρέκλα = subject form
• την καρέκλα = object/prepositional form here

This is one of the basic case changes in Greek.

Καρέκλα is a feminine singular noun, so in the accusative singular it takes τη(ν) as the article.

So:

• nominative: η καρέκλα
• accusative: την καρέκλα

You may also see τη instead of την in some contexts, especially in speech or less formal writing, but την καρέκλα is completely standard and very common.

κλειδί is a neuter noun.

That is why it takes:

• το κλειδί

In Greek, nouns have grammatical gender, and the article must match:

• masculine: ο
• feminine: η
• neuter: το

So here:

• το κλειδί = the key
• η καρέκλα = the chair

Yes, it matters, because the article and sometimes other words must agree with the noun’s gender.

A rough pronunciation guide is:

to klee-THEE moo EE-ne ka-TO a-PO tin ka-RE-kla

A few useful notes:

• κλειδί is stressed on the last syllable: κλει-ΔΙ
• είναι is stressed on the first syllable: ΕΙ-ναι
• κάτω is stressed on the first syllable: ΚΑ-τω
• από is stressed on the second syllable: α-ΠΟ
• καρέκλα is stressed on the second syllable: κα-ΡΕ-κλα

Also, δ in modern Greek sounds like the th in this, so κλειδί ends with that sound before ee.

Yes. Greek word order is more flexible than English, although the given sentence is the most neutral and natural way to say it.

Neutral order:

• Το κλειδί μου είναι κάτω από την καρέκλα.

You might also hear:

• Κάτω από την καρέκλα είναι το κλειδί μου.

That version puts more focus on the location: Under the chair is my key.

So the original sentence is the best basic pattern to learn, but Greek can move things around for emphasis.

Yes, but it changes the feel.

• Το κλειδί μου = my key (normal, everyday)
• Το δικό μου κλειδί = my own key / my key specifically

The version with δικό μου is more emphatic, as if you are contrasting it with someone else’s key.

So unless you want emphasis, το κλειδί μου is the more natural choice.

Here is the breakdown:

• Το = the
• κλειδί = key
• μου = my
• είναι = is
• κάτω από = under
• την = the (feminine accusative singular)
• καρέκλα = chair

So literally, the structure is:

• The key my is under the chair

But the natural English translation is:

• My key is under the chair.