αυτό

In a sentence

• Αυτό είναι ένα βιβλίο.

  This is a book.

  In this sentence, 'αυτό' is a demonstrative pronoun used to refer to an unspecified object, meaning 'this'.

• Θέλω να καταλάβω αυτό που λες.

  I want to understand what you are saying.

  Here, 'αυτό' is used as a demonstrative pronoun to introduce a relative clause, meaning 'that' or 'what'.

• Αυτό το βράδυ θα μείνω σπίτι.

  This evening I will stay home.

  In this usage, 'αυτό' acts as a demonstrative adjective, modifying the noun 'το βράδυ' (the evening) to mean 'this' evening.