Two forms of verb for same time (Pretérito Subjuntivo)

Question

Why are the two forms of a verb for certain times and how to use them?

For instance taking the word echar, for Pretérito Imperfecto Subjuntivo Activo my dicctionary gives

Pretérito Imperfecto Subjuntivo Activo

yo echara, echase   | nosotros echáramos, echásemos 
tú echaras, echases | vosotros echarais, echaseis 
él echara, echase   | ellos echaran, echasen

Obviously there are two forms, e. g. in Sg. 1.P. echara and echase.

Is there any difference in meaning or usage?
What is to be used preferably?
Do the answers to the above questions apply to all compounded times, which also allow two forms, which is supposedly linked to the auxiliar verb used in them?

The name of and rescources concerning this matter are desired, too. Here are the other times:

Pretérito Pluscuamperfecto Subjuntivo Activo

yo hubiera (hubiese) echado   | nosotros hubiéramos (hubiésemos) echado 
tú hubieras (hubieses) echado | vosotros hubierais (hubieseis) echado 
él hubiera (hubiese) echado   | ellos hubieran (hubiesen) echado 

Pretérito Imperfecto Subjunctivo Pasivo

yo fuera (fuese) echado   | nosotros fuéramos (fuésemos) echados 
tú fueras (fueses) echado | vosotros fuerais (fueseis) echados 
él fuera (fuese) echado   | ellos fueran (fuesen) echados 

Pretérito Pluscuamperfecto Subjuntivo Pasivo

yo hubiera (hubiese) sido echado   | nosotros hubiéramos (hubiésemos) sido echados 
tú hubieras (hubieses) sido echado | vosotros hubierais (hubieseis) sido echados 
él hubiera (hubiese) sido echado   | ellos hubieran (hubiesen) sido echados

score 5 · Answer 1 · edited Jun 17 '20 at 09:53

This phenomenon only happens in the pretérito imperfecto de subjuntivo; the other verbal forms you mention are, in fact, variations of this (the pluscuamperfecto is formed using the imperfecto form of the auxiliary verb haber, after all).

Originally, only the -se form had this value, while the -ra form was the pretérito pluscuamperfecto de indicativo by itself. This is; in the XVI century:

yo echase -> pretérito imperfecto de subjuntivo

yo echara -> pretérito pluscuamperfecto de indicativo, equivalent to nowadays "yo había echado"

It happened that the pluscuamperfecto came to be used in conditional constructions as well as the subjunctive; and, given time, the -ra form lost its original value and became equivalent to the -se form, while the current form of pluscuamperfecto came into use. This is the reason why we have two forms nowadays.

For most practical reasons, both forms are equivalent; but, given their origins, some slight differences remain between them. Some of these slight differences are:

The -ra form can sometimes be used instead of the conditional in negative constructions (no quisiera tener que volver, equivalent to no querría tener que volver), while the -se form cannot.
The -ra form is preferred when the subjunctive has no conditional or hypotetical meaning: I'd say le dije que se fuera rather than le dije que se fuese, for instance, but I'd prefer me gustaría que vinieses to me gustaría que vinieras.

Nevertheless, both forms are generally interchangeable in modern Spanish. It is up to you to decide which one to use in most cases.

Two forms of verb for same time (Pretérito Subjuntivo)

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