If only means the same as I wish.  We can use if only instead of I wish with the same use of tenses.
Examples:
     If only I was/were as beautiful as you.
     If only I hadn’t dropped the meat.
     If only Sandra would tell me the truth. 

If only can be more emphatic than wish.  It often expresses regret.
Example:
     If only you'd told me you had an extra toaster.  I wouldn't have bought a new one. 

After if only we can soemtimes use the present tense in a wish about the future.
Example:
     If only the train gets in on time, we'll just catch the three o'clock bus.

Only can sometimes be in mid position:  If you'd only told me, I could have eaten it.