Quando o objeto vem logo depois usa-se your, quando não vem usa-se yours.
This is your car.
This car is yours.

Exactly, where it is followed by an object, you use "your". You need to add extra information so it makes sense.

Example:

She is "your" mom.
Ela é a sua mai.


But if you want to use "yours". It is mostly placed at the end of the sentence.

The cat is "yours".
O gato é seu.

Thanks dude