The second conditional is formed by if + past simple, ...would + infinitive
To talk about things in the future that are probably not going to be true.
Secondly, we can use it to talk about something in the present which is impossible, because it's not true.
* We can use 'were' instead of 'was' with 'I' and 'he/she/it'. This is mostly done in formal writing.
How is this different from the first conditional?
This kind of conditional sentence is different from the first conditional because this is a lot more unlikely.
Second conditional: If I had enough money I would buy a house with twenty bedrooms and a swimming pool. (I'm probably not going to have this much money, it's just a dream, not very real)
First conditional: If I have enough money, I'll buy some new shoes.
(It's much more likely that I'll have enough money to buy some shoes.
Another example:
Second conditional: If she studied harder, she would pass the exam.
(I think that she won't study harder, or it's very unlikely, and so she won't pass).
First conditional: If she studies harder, she'll pass the exam.
(I think it's possible she will study harder and so she'll pass)