The imperfect subjuntive has two forms. For example:
- Ojalá viniera.
- Ojalá viniese.
I think both has the same meaning. However, is there any subtle difference?
The advice I was always given was that there is no difference in the meaning, but to stick to one of them (ie don't mix and match) in a conversation/piece of text.
There is no difference in meaning. I have found that the -ra endings are more common in Latin American Spanish and the -se endings are more common in European Spanish. So I would use the one that represents the type of Spanish you are trying to speak.