The Accusative case is used with direct objects. Read more about Subject – Object distinction here.
In Russian not only pronouns can be subjects or objects, but also nouns (later you will learn that adjectives as well). So if there is something or somebody that we love, hate, adore, ignore, see, listen to, know, understand etc. – they are our objects. They should be in the accusative case.
For example, Steven is a person who makes an action. He is a subject. His car doesn’t do anything. It does not make any action but it accepts the Steven’s action. So we can call it a direct object. So the car (машина) should be in the accusative case (машину), which means we change its ending. Feminine nouns have the ending -у or -ю in the accusative case. And we also change masculine animated nouns. Saying animated I mean a person or an animal. For example,
- Я знаю президента – I know the president. («President» is a masculine animated noun, the ending is -a).
- Я вижу дом. – I see the house («House: is a masculine inanimate noun. That is why it hasn’t been changed)