Χτυπάω ελαφρά την πόρτα, γιατί το μωρό κοιμάται.

I knock lightly on the door because the baby is sleeping.

Breakdown of Χτυπάω ελαφρά την πόρτα, γιατί το μωρό κοιμάται.

• γιατί: because
• κοιμάμαι: to sleep
• η πόρτα: the door
• το μωρό: the baby

Questions & Answers about Χτυπάω ελαφρά την πόρτα, γιατί το μωρό κοιμάται.

Why is it την πόρτα and not just πόρτα?

Because Greek normally uses the definite article much more often than English.

• την πόρτα = the door
• πόρτα by itself = door / a door, depending on context

Here, την is the feminine singular accusative form of the, because πόρτα is:

• feminine
• singular
• the direct object of χτυπάω (I knock/hit)

So:

• Χτυπάω την πόρτα = I knock on the door

In natural Greek, leaving out the article here would usually sound incomplete or unnatural.

Why is την used specifically? How do I know it should be accusative?

Because την πόρτα is the thing being affected by the verb.

In this sentence:

• Χτυπάω = I knock / I hit
• την πόρτα = the object receiving the action

That makes πόρτα a direct object, so Greek uses the accusative case.

For this noun, the forms are:

• nominative: η πόρτα = the door (subject)
• accusative: την πόρτα = the door (object)

Compare:

• Η πόρτα είναι ανοιχτή. = The door is open.
  (η πόρτα is the subject)

• Χτυπάω την πόρτα. = I knock on the door.
  (την πόρτα is the object)