Wishes and Regrets.

Expressing a wish:

Form: 

If only / I wish + simple past

e.g: 

If only I knew how to use a computer. (I don’t know how to use a computer and I would like to learn how to use it)

Use:

• To express a wish in the present or in the future.
• The simple past here is an unreal past.
• When you use the verb to be the form is “were”.
  Example:
  I wish I were a millionaire!

Expressing a regret:

Form: 

If only / I wish + past perfect

e.g: If only I had woken up early. (I didn't wake up early and I missed my bus.)

Use:

• To express a regret.
• The action is past.

Link: http://www.english-in-chester.co.uk/e-learning/lesson/wishes-regrets/