Όποια λέξη δεν καταλαβαίνεις, γράψε την στο τετράδιό σου.

Any word that you don't understand, write it in your notebook.

Breakdown of Όποια λέξη δεν καταλαβαίνεις, γράψε την στο τετράδιό σου.

• δεν: not
• σου: your
• καταλαβαίνω: to understand
• σε: in

Questions & Answers about Όποια λέξη δεν καταλαβαίνεις, γράψε την στο τετράδιό σου.

What exactly does Όποια mean here, and why is it in that form?

Όποια comes from the pronoun όποιος / όποια / όποιο, which means whoever / whichever / any … that.

• όποια is:
  • feminine
  • singular
  • accusative case

It has to agree with λέξη (word), which is also feminine singular accusative (τη λέξη).

So Όποια λέξη literally means whichever word / any word that… in the sense of any word that you don’t understand.

Why is λέξη in the accusative case?

In this structure, λέξη is the direct object of the verb καταλαβαίνεις (you understand):

• (Δεν) καταλαβαίνεις (ποια;) λέξη → (You don’t understand which word?)

Direct objects in Greek take the accusative case, so λέξη must be accusative.
The phrase Όποια λέξη δεν καταλαβαίνεις is like saying Any word you don’t understand.

What is the role of δεν, and why does it come before καταλαβαίνεις?

δεν is the standard negation particle in Greek for most verbs in the indicative mood.

• The normal order is: δεν + verb
  
  • δεν καταλαβαίνεις = you don’t understand

So Όποια λέξη δεν καταλαβαίνεις = Any word (that) you don’t understand.
Putting δεν after the verb would be wrong in standard Greek.