"He is not your uncle."
Translation:Αυτός δεν είναι ο θείος σου.
Something is confusing me and i would appreciate any help: Isn't "your uncle" in the accusative? Why is the form used in greek a nominative one? Why not "του θεού"?
I understand why you might think accusative is needed here, but with the verb είναι objects always stay in the nominative case (you can think of it as an equal sign, because the nouns on both sides refer to the same thing and thus are in the same case).
As for the genitive case, it makes no sense to say του θέιου here; remember that the genitive case is used for the "possessor" (here σου, the genitive of εσύ meaning "of you") and not for the object being "possessed" (here ο θείος)
Αυτός δεν είναι ο θείος = he is not the uncle: θείος in nominative
Αυτός δεν είναι ο θείος σου = he is not your uncle ("the uncle of you"), θείος still in nominative
You could use του θείου in a sentence where "uncle" is the possessor:
Αυτός δεν είναι ο σκύλος του θείου = it is not the uncle's dog ("the dog of the uncle")
You can even stack genitive nouns/pronouns:
Αυτός δεν είναι ο σκύλος του θείου σου = it is not your uncle's dog ("the dog of the uncle of you")
Thank you very much for your answer! I am sorry i mixed the genitive and accusative, which is why i said "του θείου". Anyway, i would like to ask about the verbs which objects stay in the nominative case? How can i know them? Verbs έχω/βοηθώ... do not have this equality you spoke of but still have their objects in the nominative? Please help if you can. Thanks again.
The direct object of a verb is always in the accusative case regardless of the verb, however this doesn't apply to είμαι, because it simply doesn't have a "true" object:
In the sentence "he is your uncle", "he" isn't doing anything to "your uncle", so there's no reason to use the accusative case. Instead, "is" is just a "link" (known as a "copula") here to show that "he" and "your uncle" refer to the same thing, but it doesn't describe any action.
Another common copula verb is γινώ "to become":
Θέλω να γινώ (ένας) γιατρός = I want to become a doctor (γιατρός is in nominative here)
With other "transitive" verbs that describe an action done to an object, like έχω and βοηθώ, you always have to use the accusative for the object, otherwise it will be ungrammatical.
Βοηθώ τον θείο σου = I help your uncle
Only that become is mediopassive in Greek (i.e. the passive form is used as present). Therefore
γίνομαι ( I become) is used as a starting point since Greek does not have infinitive
(γινώ does not exist, the dependent form is γίνω—>θα γίνω, να γίνω)
Otherwise I find your explanation very good and γίνομαι καπνός
English does the same thing with the verb "to be". So, we say: "Honey is sweet." using the adjective "sweet" but "She answered me sweetly." using the adverb. "Susan is angry." "He replied angrily."