Κατά λάθος άφησα τα κλειδιά στο αυτοκίνητο.

Yes. κατά λάθος is a fixed expression meaning by mistake or accidentally.

• κατά is a preposition
• λάθος means mistake

But in this phrase, learners should usually treat κατά λάθος as one chunk with the meaning accidentally.

Examples:

• Το έκανα κατά λάθος. = I did it by mistake.
• Πάτησα το κουμπί κατά λάθος. = I pressed the button accidentally.

άφησα is the 1st person singular aorist of αφήνω = to leave.

So:

• αφήνω = I leave / I am leaving
• άφησα = I left

In this sentence, the speaker is talking about a completed action in the past, so Greek uses the aorist:

• Κατά λάθος άφησα... = I accidentally left...

A few forms:

• άφησα = I left
• άφησες = you left
• άφησε = he/she/it left
• αφήσαμε = we left

Yes. Greek very often omits subject pronouns when they are not needed.

The verb ending in άφησα already tells you the subject is I, so εγώ is unnecessary.

• άφησα already means I left
• εγώ άφησα can be used for emphasis, like I left, not someone else

So:

• Κατά λάθος άφησα τα κλειδιά στο αυτοκίνητο. = neutral
• Εγώ άφησα τα κλειδιά στο αυτοκίνητο. = more emphatic

τα κλειδιά is the direct object of the verb άφησα, so it is in the accusative case.

Here:

• τα = the for neuter plural
• κλειδιά = keys

The noun is:

• singular: το κλειδί = the key
• plural: τα κλειδιά = the keys

In Greek, the article changes with gender, number, and case. Here κλειδιά is neuter plural, so the article is τα.

Because κλειδί is a neuter noun whose plural is κλειδιά.

Pattern:

• το κλειδί = the key
• τα κλειδιά = the keys

This is a common noun pattern in Greek. Other examples are similar:

• το παιδί → τα παιδιά
• το τραγούδι → τα τραγούδια

So κλειδιά is simply the normal plural form.

στο is a contraction of σε + το.

• σε usually means in / at / to / on, depending on context
• το = the for a neuter singular noun

So:

• σε το αυτοκίνητο becomes στο αυτοκίνητο

This contraction is standard in modern Greek.

Similar examples:

• στο σπίτι = in/at the house
• στο γραφείο = in/at the office
• στο τραπέζι = on the table

Because the exact meaning of σε depends on context.

In Greek, σε can cover several ideas that English separates:

• in
• at
• on
• sometimes to

In this sentence, with leave the keys, the natural interpretation is in the car:

• άφησα τα κλειδιά στο αυτοκίνητο = I left the keys in the car

If you want to make inside extra explicit, you can say:

• μέσα στο αυτοκίνητο = inside the car

But in everyday Greek, στο αυτοκίνητο is perfectly natural here.

Yes. μέσα στο αυτοκίνητο means inside the car and is a bit more explicit.

Compare:

• άφησα τα κλειδιά στο αυτοκίνητο = I left the keys in the car
• άφησα τα κλειδιά μέσα στο αυτοκίνητο = I left the keys inside the car

The version without μέσα is already very natural and common.

The word order can change. Greek is more flexible than English because the endings and articles help show the grammar.

The given sentence is very natural:

• Κατά λάθος άφησα τα κλειδιά στο αυτοκίνητο.

But you could also say:

• Άφησα κατά λάθος τα κλειδιά στο αυτοκίνητο.
• Τα κλειδιά τα άφησα κατά λάθος στο αυτοκίνητο.
• Στο αυτοκίνητο άφησα κατά λάθος τα κλειδιά.

These versions can shift emphasis:

• beginning with κατά λάθος emphasizes the mistake
• beginning with τα κλειδιά emphasizes the keys
• beginning with στο αυτοκίνητο emphasizes the location

Sometimes yes, but the meaning changes slightly.

• άφησα τα κλειδιά στο αυτοκίνητο = I left the keys in the car
• ξέχασα τα κλειδιά στο αυτοκίνητο = I forgot the keys in the car

In many real situations, both could work, but the focus is different:

• άφησα focuses on the act of leaving them there
• ξέχασα focuses on forgetting

So if the English idea is specifically I left the keys in the car, then άφησα is the best match.

A simple pronunciation guide is:

Ka-TA LA-thos A-fi-sa ta kli-DYA sto af-to-KI-ni-to

A few notes:

• θ in λάθος sounds like th in think
• δ would sound like th in this, but there is no δ here
• χ would be a rough sound, like German Bach, but again not in this sentence
• αι is pronounced like e
• ει is pronounced like ee
• αυ in αυτοκίνητο here sounds like af, because it comes before a voiceless consonant

Stress matters:

• κατά
• λάθος
• άφησα
• κλειδιά
• αυτοκίνητο

Yes, accents matter because they show which syllable is stressed.

In Greek, every multi-syllable word normally has a written stress mark. That helps you pronounce the word correctly:

• λάθος → stress on λά
• άφησα → stress on ά
• κλειδιά → stress on διά
• αυτοκίνητο → stress on κί

If you stress the wrong syllable, people may still understand you, but it will sound unnatural and can sometimes cause confusion.

It functions as an adverbial expression.

Even though it is made of two words, in the sentence it works like an adverb modifying the verb:

• άφησα = I left
• κατά λάθος άφησα = I accidentally left

So it tells you how the action happened.

Yes, it is completely natural everyday Greek.

A Greek speaker would easily say:

• Κατά λάθος άφησα τα κλειδιά στο αυτοκίνητο.

It sounds normal, conversational, and idiomatic. It is a very useful pattern you can reuse:

• Κατά λάθος άφησα το κινητό στο σπίτι.
• Κατά λάθος έστειλα το μήνυμα σε λάθος άτομο.
• Κατά λάθος έκλεισα τον υπολογιστή.

So this is a good sentence to learn as a model.