The second conditional is used to talk about imaginary or impossible situations.

How to form sentences using the second conditional :

♦ Affirmative form : If + Subject + past simple, Subject + would + infinitive
or Subject + would + infinitive, If + Subject + past simple

Examples :

If I won the lottery, I would be very happy. —> Imaginary situation.
I would be very happy if i won the lottery.

If I saw a ghost, I would be scared. —> Impossible situation.
I would be scared if I saw a ghost.

♦ Negative form : If + Subject + past simple, Subject + wouldn’t + infinitive
or Subject + wouldn’t + infinitive, If + Subject + past simple

Examples :

If I won the lottery, I wouldn’t be very happy.
I wouldn’t be very happy if I won the lottery.

♦ Question form : If + Subject + past simple, Question word (how, what, …) + would + subject + infinitive
or Question word (how, what, …) + would + subject + infinitive, If + Subject + past simple

Examples :

If you won the lottery, what would you do ?
What would you do if you won the lottery ?