Άκουσα το χτύπημα.

Άκουσα is the 1st person singular aorist form of ακούω, meaning I heard.

In this sentence, it refers to a single completed event in the past: you heard something once or at a particular moment.

So:

• ακούω = I hear / I am hearing
• άκουσα = I heard

Greek aorist is often the tense used for a simple past action in English.

Greek often leaves out subject pronouns when they are not needed.

The ending of άκουσα already tells you the subject is I, so εγώ is unnecessary.

That means:

• Άκουσα το χτύπημα. = I heard the knock.
• Εγώ άκουσα το χτύπημα. = I heard the knock with extra emphasis on I

English usually requires the pronoun, but Greek often does not.

Το is the definite article, meaning the.

So το χτύπημα means the knock / the hit / the sound rather than just a knock or knocking in a general sense.

Greek uses the definite article very naturally, and here it shows that the speaker means a specific sound or event.

It is the direct object of άκουσα, so grammatically it is in the accusative case.

However, this can be confusing because χτύπημα is a neuter singular noun, and in the neuter singular the nominative and accusative look the same.

So:

• nominative: το χτύπημα
• accusative: το χτύπημα

The form does not change, but its role in the sentence does.

Χτύπημα comes from χτυπάω / χτυπώ, which can mean hit, strike, knock, beat, or ring depending on context.

So χτύπημα can mean things like:

• a hit or blow
• a knock
• a strike
• the sound of something hitting

In a sentence like Άκουσα το χτύπημα, it often means something like the knock or the sound of the impact, depending on the situation.

A good approximation is:

AH-koo-sa to KH-TEE-pee-ma

More closely:

• Άκουσα = AH-koo-sa
• το = to
• χτύπημα = KH-tee-pee-ma

A few pronunciation notes:

• The stress is on Ά in Άκουσα
• The stress is on τύ in χτύπημα
• χ is not the English ch of chair
• It is a rough sound, like the ch in Scottish loch or German Bach

In Modern Greek, accent marks are normally still written on capitalized words when needed.

So Άκουσα keeps its accent to show where the stress falls.

This is very helpful for learners, because stress is important in Greek pronunciation.

Greek word order is more flexible than English.

Άκουσα το χτύπημα is the most neutral and natural order here: verb + object.

But other orders are possible for emphasis. For example:

• Το χτύπημα άκουσα. = emphasis on the knock
• Εγώ άκουσα το χτύπημα. = emphasis on I

So the original sentence is the neutral way to say it, but Greek can move words around more easily than English.

This is a very common question.

• άκουσα = I heard
  A single completed event
• άκουγα = I was hearing / I used to hear
  Ongoing, repeated, or background action in the past

So:

• Άκουσα το χτύπημα. = I heard the knock.
• Άκουγα χτυπήματα όλη νύχτα. = I was hearing knocks all night.

The sentence you were given uses άκουσα because it presents the hearing as one completed event.

In most cases, άκουσα is best translated as I heard.

Greek aorist usually corresponds to the simple past in English, not automatically to I have heard.

So for this sentence, the most natural English match is:

• Άκουσα το χτύπημα. = I heard the knock.

Sometimes English translation depends on context, but if you are learning the basic value of the form, think of άκουσα as I heard.