Second declension nouns can either be masculine, feminine or neuter in gender. Masculine and feminine second-declension nouns share case endings. Because of this, there are many that are either feminine or masculine in gender, with only the use of article to distinguish them (e.g. ὁ/ἡ θεός, the god/goddess).
* To decline any noun, you should take the genitive singular form (which can be found in a proper dictionary entry) and remove the ending; to that stem add the appropriate ending for the case and number required.
Unlike first declension nouns, the plural genitive of second declension nouns does not always take a circumflex on the ultima. But as with first declension nouns, when the accent of the genitive singular is on the ultima, both the genitive and dative will take a circumflex accent, singular and plural. In addition, the plural declension marker -οι is always counted as short for the purpose of determining accent.