Obligaciones Puras y Simples: Definición
Es aquella que no está sometida a ningún tipo de pluralidad. (Son aquellas cuyo cumplimiento no depende de la ocurrencia o de ningún acontecimiento o modalidad, Deben cumplirse de inmediato). El vínculo de la misma es plenamente exigible desde el comienzo de su obligación.

March | 2016 | Léxico Jurídico Español-Inglés - Rebecca Jowersrebeccajowers.com › 2016/03
https://rebeccajowers.com/2016/03/
Obligaciones puras (“unconditional obligations”) are those that are not ... and English are simply legal synonyms that are not always clearly distinguishable, ...
Puro y simple isn’t so pure and simple!
In legal contexts puro/a (y simple) cannot generally be translated literally as “pure (and simple),” since the expression often means “unconditional.” Obligaciones puras (“unconditional obligations”) are those that are not subject to any condition or term (no sujetas a ningún tipo de condición, plazo o término) and, thus, payment or performance of those obligations may be demanded at any time. Donación pura y simple denotes an “unconditional gift,” in contrast to donación modal (or) donación onerosa upon which the donor has imposed a condition or encumbrance. With regard to negotiable instruments (títulos valores), checks and bills of exchange (cheques y letras de cambio) must contain an “unconditional order to pay a fixed amount of money.” In Spanish this is expressed as un mandato puro y simple de pagar una suma determinada. And heredero puro y simple denotes an heir who accepts his inheritance unconditionally (known as aceptación pura y simple), without exercising the option to limit his liability for the debts of the inherited estate to the value of the inheritance (called beneficio de inventario).

Here it uses "pure and unconditional", but I think "unconditional" on its own will suffice.
Modalidades de las obligaciones: características y ejemplos ...www.lifeder.com › Derecho
https://www.lifeder.com/modalidades-obligaciones/
Lo primero es distinguir inicialmente entre obligaciones puras y modales ... Las obligaciones puras se entienden en contraste con las obligaciones modales, puesto que las obligaciones puras y simples ... Pure and conditional obligations.

--------------------------------------------------
Note added at 1 day 11 hrs (2020-07-27 02:00:36 GMT) Post-grading
--------------------------------------------------

Thank you.