What are the rules of 2 verbs combination?

Question

Sometimes 2 verbs are connected without any preposition (not only modal verbs): planeaban cambiar; te permitian trabajar; esperaba comprar; deseaba regresar; decidieron volver; paso hacer; no sabe nadar; deja pasar

Sometimes the preposition "de" is used for connection: trataba de vender; no ceso de decir; dejas de vivir; acabo de hablar

And more fliquently they are combined by "a" aspiro a buscar; iba a despedirse; aprendías a cantar; les enseñaban a nadar.

Answer 1

You´re right.

And with the combination of dejar and vivir, the combination varies, ie "¡Dájame vivir en paz!" (imperative Tú)

There is no rule that I know of other than memorization - although it may exist somewhere hidden within the R A E.