' If only ...', is hypothetical - in effect you're saying, 'I wish something were true', (but in reality, it probably isn't). E.g., If only we had more time, we could practise this much more. [we don't have more time, but we wish we did] 'Only if ...', is a conditional. It means that something can happen 'only if' something else happens or is true. E.g. 'We can practise this more, only if we have more time.' [We are not talking about how likely or unlikely having more time is. It is just a fact that we can ONLY practice more IF we have more time] 'If only ... ', usually used with 'wishes' and 'regrets'. It is basically used for emphasis. 'Only if ...', a very important conditional clause. We must use this if we want to say something is entirely dependent on something else happening. This kind of logical statement is often used in mathematics and computer programming. 'If only ...' is more likely to be used in poetry :) Let me know if you want more examples.

If only you would shut up = I wish you would shut up If only I hadn't done that = I wish I hadn't done that Only if you shut up will I help you = only on that condition.... Only if you promise to love me forever will I agree to marry you. You get the picture...