Two types of imperfect subjunctive.
I know there are two types of imperfect subjunctive ra and se.
For example, hubiera and hubiese.

Why there are two types in imperfect subjunctive?
When to use ra imperfect subjunctive and when to use se imperfect subjunctive?

Question

Two types of imperfect subjunctive.
I know there are two types of imperfect subjunctive ra and se.
For example, hubiera and hubiese.

Why there are two types in imperfect subjunctive?
When to use ra imperfect subjunctive and when to use se imperfect subjunctive?

Answer

En general, yo no me preocuparía demasiado por este tema. Pocos nativos lo hacen. No obstante, si tienes verdadera curiosidad, puedes echar un vistazo a todo esto.
http://www.wikilengua.org/index.php/Pret%C3%A9rito_imperfecto_de_subjuntivo
http://www.hispanoteca.eu/Foro-preguntas/ARCHIVO-Foro/Imperfecto%20subjuntivo.htm
http://www.hispanoteca.eu/Gram%C3%A1ticas/Gram%C3%A1tica%20espa%C3%B1ola/Imperfecto%20de%20subjuntivo%20-%20formas%20-ra%20y%20-se.htm
http://cvc.cervantes.es/foros/leer_asunto.asp?vCodigo=40018

Answer

I don't have an answer for why there are two types, but I know both are valid in any situation.

Answer

La forma en -RA procede del pluscuamperfecto de indicativo latino,  La forma en -SE es verdaderamente subjuntiva y procede del pluscuamperfecto de subjuntivo latino.

Answer

Tienen el mismo significado. Subjuntivo imperfecto.