Unreal tenses

Unreal tenses are verb forms used in subordinate clauses that express unreal or hypothetical situations:

• The past subjunctive, which has the same form as the past simple tense, is used to refer to present or future time:
  

  I wish you were here. (you are not here)
  What would you do if you won the lottery? (you probably won't win the lottery)

• The past perfect subjunctive, which has the same form as the past perfect tense, is used to refer to past time:
  

  If I had gone to bed earlier, I wouldn't have overslept. (I didn't go to bed earlier)
  He seems as if he hadn't slept for days. (it seems that he hasn't slept for days, but actually he has)

Unreal tenses are used in second and third conditionals and after wish / if only, as if / as though, it's (high) time and would rather / would sooner.