Accusative for direct objects
In a sentence, there is usually a subject -- a producer of the action, and there is an object -- a person or a thing which the action is made to.
Она любит его. (She loves him) -- She (subject) loves (verb) him (direct object).
A direct object is something that you love, give, take, know, buy, want or sell. Direct objects should be in the accusative case. All feminine nouns are changed, despite whether or not they are animated, but only masculine animated nouns are changed (meaning that inanimate masculine nouns do not decline).
Plural nouns are changed only if they refer to people or animals. In this case the endings are the same as in the genitive case. See more about cases here.