This & That: αυτός, εκείνος

αυτός (“this”) and εκείνος (“that”) are adjectives on the -ος / -η / -ο pattern and agree with their noun in gender, number and case: αυτός / αυτή / αυτό, εκείνος / εκείνη / εκείνο, plural αυτοί / αυτές / αυτά. The one genuinely new rule is the obligatory double article: a noun modified by a demonstrative keeps its definite article - αυτό το σπίτι (literally “this the house”), εκείνη η πόρτα, αυτά τα παιδιά. All three words - demonstrative, article and noun - agree together. Dropping the article («αυτό σπίτι») is the classic beginner mistake and is simply ungrammatical. Standing alone, with no noun after it, the demonstrative needs no article at all: Αυτό είναι το σπίτι μου.