What olik-olik wrote is correct, but not complete. Her first example works only with certain verbs. 
I wish I had a million dollars.
I wish I were teaching English again.
I wish I could fly.   (could + infinitive)
I wish I knew the answer to your question.
 
These examples of the present describe situations rather than actions. It doesn �t work with actions:
 
X   I wish the police found the suspect quickly.
X   I wish I finished the assignment.
X   I wish you told me about that.
 
I hope this helps, without confusing the matter.
Bruce