Past Unreal Conditional.

1. The past unreal is used for express past situations that could have been different, in different conditions. 
Example: 
If I had driven to my house, I would have arrived earlier that night. 

2. The structure of past unreal is a dependant clause (the "if" clause) that is conformed by past perfect, plus a main clause conformed by would (or other modals as  could or might) plus have as an auxiliary plus past participle.

If + past perfect (then) would have + past participle.

3. In spoken english (informally speaking, actually) is common to hear contractions in past conditional.
Here we have a video about the appropiate pronunciation of this conditionals.