I don't understand when i have to use δικες, δικο, δική and etc. Is there any rule that i can memorize easily?
To translate possessive pronouns (mine, yours, etc) in Greek, you need to put the adjective δικός/δική/δικό and the short time of the possessive pronoun in Greek (μου, σου, του, etc). Note that the adjective δικός is declined like all the other adjectives (it has singular and plural number, cases and genders). For example:
my book = το βιβλίο μου
This book is mine. = Αυτό το βιβλίο είναι δικό μου.
However, the adjective δικός can be also used in the first case to give emphasis. :-)