Greek nouns are volatile. Their form and spelling change according to their case.
A case represents the noun’s role and function in a sentence. Modern Greek has four cases: the nominative (ονομαστική), the genitive (γενική), the accusative (αιτιατική) and the vocative (κλητική).
The nominative mainly replies to “Who?” and is the subject of the verb.
The genitive has a lot of functions. The most common are ownership, quality and purpose: “Jane’s books”, “oak wood”, “wine glass”.
The accusative usually follows a preposition: “I’m talking to Jane”, “I’m dining with John.” “A letter from Anne.” It can’t be the verb subject.
The vocative is the one you use when you say “Hello, Anna!”. You probably won’t need to use it for anything other than proper names. If you do, you probably are intermediate or advanced level and you’ll know all about cases already!
Cases are important to know. First, so that you can sound like a native speaker and second, because the word order in Greek sentences is fluid and verb subjects can be found at various positions, causing misunderstandings.