Χτύπησα τον αγκώνα μου στην πόρτα και τώρα πονάει λίγο.

Χτύπησα is the aorist of χτυπάω / χτυπώ, so it means I hit / I banged in the sense of a single completed action.

Greek often drops subject pronouns when they are clear from the verb ending. Here, the ending -σα already tells you the subject is I, so εγώ is not needed.

Not necessarily. In this kind of sentence, χτύπησα τον αγκώνα μου στην πόρτα usually means I banged my elbow on the door, very often accidentally.

So although χτυπάω can mean hit/strike, in everyday contexts with body parts it often has the natural English sense of bang or knock.

Greek normally uses the pattern definite article + noun + possessive clitic:

τον αγκώνα μου = my elbow

So even though μου means my, the article τον is still normally there. This is very common in Greek:

• το χέρι μου = my hand
• η μητέρα μου = my mother
• τον φίλο μου = my friend

English does not do this, but Greek usually does.

Because αγκώνας is the direct object of χτύπησα.

The dictionary form is:

• ο αγκώνας = the elbow (nominative)

But after I hit, Greek uses the accusative:

• τον αγκώνα = the elbow (accusative)

This is a regular pattern for many masculine nouns in -ας:

• ο φίλος → τον φίλο
• ο άνθρωπος → τον άνθρωπο
• ο αγκώνας → τον αγκώνα

Στην is the contraction of σε + την.

• σε = a preposition that can mean in, on, at, to
• την = the (feminine accusative singular)

So:

• σε την πόρτα → στην πόρτα

In this sentence, στην πόρτα means something like on the door or against the door.

It is in the accusative, but for many feminine nouns in -α, the nominative and accusative singular look the same.

So:

• η πόρτα = nominative
• την πόρτα = accusative
• στην πόρτα = σε + την πόρτα

The case is shown mainly by the article, not by a visible change in the noun itself.

Because πονάει is third person singular: it hurts / he hurts / she hurts.

Here the understood subject is the elbow or simply it:

• Ο αγκώνας μου πονάει. = My elbow hurts.
• Τώρα πονάει λίγο. = Now it hurts a little.

If you said πονάω, that would mean I hurt / I am hurting.

Greek often leaves out subject pronouns when the meaning is already clear.

So instead of saying the equivalent of it hurts, Greek can simply say:

πονάει

The listener understands that the thing hurting is the elbow, because that was just mentioned.

Here λίγο means a little / a bit, and it works as an adverb, modifying πονάει:

• πονάει λίγο = it hurts a little

Putting λίγο at the end is very natural and neutral. Greek word order is fairly flexible, but this position sounds especially normal in everyday speech.