The second conditional is used to refer to an imaginary situation now or in the future.
'If Anna was here, she would know what to do.' (but she is not here at the moment, so she cannot help) Here we are talking about the present and imagining a situation that is different from reality.
'If I lost my job tommorrow, I would move to London.' Here we are talking about a possible event in the future, but by using the second conditional we make it clear that we don't really think it will happen.
The third conditional is used to imagine how a situation in the past could have been different.
'If he had not gone to the casino, he would not have lost all his money.' (In reality he went to the casino and lost all his money) If what actually happened was positive we use negative to imagine a different situation. If what actually happened was negative, we use positive.
'If we had increased our prices, we would not have sold all the clothes.' (In reality we didn't increase our prices, so we sold all the clothes)
Second conditional = imaginary situation now or in the future.
Third conditional = imaginary situation in the past.