Theory and examples: 1st and 2nd conditionals.

 The Second Conditional compared to the First Conditional.

 The second conditional uses the past simple after if, then 'would' and the infinitive:

if + past simple, ...would + infinitive

(We can use 'were' instead of 'was' with 'I' and 'he/she/it'. This is mostly done in formal writing).

It has two uses:

Ø  First, we can use it to talk about things in the future that are probably not going to be true. Maybe I'm imagining some dream, for example.

o   If I won the lottery, I would buy a big house.(I probably won't win the lottery)

o   If I met the Queen of England, I would say hello.

o   She would travel all over the world if she were rich.

o   She would pass the exam if she studied. (She never studies, so this won't happen)

Ø  Second, we can use it to talk about something in the present which is impossible, because it's not true.

 Have a look at the examples:

o   If I had his number, I would call him. (I don't have his number now, so it's impossible for me to call him).

o   If I were you, I wouldn't go out with that man.

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.

For example in the second conditional:

o   If I had enough money I would buy a house with twenty bedrooms and a swimming pool (I will probably not have this much money, it's just a dream, not very real)

But  in the first conditional:

o   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)

taken from: http://www.perfect-english-grammar.com/second-conditional.html