When you negate verbs which have a direct (as opposed to oblique) object, the object may change cases from the accusative to the genitive:
- Я пью водку / я не пью водку / я не пью водки
- Я видел сестру / я не видел сестру / я не видел сестры
In this negated form, the genitive and the accusative slightly differ in meaning: the accusative form assumes definiteness ("I didn't see the sister") while the genitive assumes indefiniteness ("I didn't see a sister", or "I didn't see any sister").
It also works for proximal possession and closely related existential clauses with быть, where the semantic patient is a grammatical subject:
- У меня есть машина (= я имею машину) / У меня нет машины (= я не имею машины)
- Вчера был дождь / вчера не было дождя.
When negating the proximal possession, the genitive is obligatory.
Oblique objects (those which are not in the accusative), indirect objects, subjects etc. do not change their case when negating verbs:
- Я избежал наказания / я не избежал наказания
- Я поверил другу / я не поверил другу
- Я говорю о тебе / я не говорю о тебе
- Он был мне другом / он не был мне другом
If I want to say "I did not use my car", would it be "Я не иcпользовал(а) мою машину"?
Использовать машину is not an idiomatic translation of "to use a car", so I'll use "I did not wash my car" for this example.
You can use either case here, but they would mean slightly different things.
Я не мыл свою машину would mean just what it says on the tin: "I didn't wash my car".
Я не мыл своей машины would mean "I didn't wash a car of mine" or "I haven't washed a car of mine".
In isolation, this sounds as peculiar in Russian as it does in English, but it could make sense in a sentence like Я вымыл тысячи чужих машин, но своей машины я не мыл ни разу "I've washed thousands of other people's cars, but I've never washed a car I would own".
