¡Hola Jim! ¿Qué tal? I will try to explain you the difference: If you use "apostar" to express: to risk money on something by trying to predict the result, the result that you say must be introduce is introduced by the preposition "a": "Te apuesto 100 euros a que esa chica es española". It is not accurate if you don't use the preposition in this case: "Te apuesto 100 euros que esa chica es española". If you use "apostarse" + "algo" + "alguien", that "alguien" must be introduced by "con": "Se apostó una cena con sus amigos a que esa chica era española". If you are betting on a race or sport, you can use "por": "He apostado por el caballo número 4." If you use "apostar" to express that you are almost certain that something is true or that something will happen, we introduce the object with "que": "Te apuesto que no lloverá hoy". Note the difference between this sentence and: "Te apuesto 100 euros a que no lloverá hoy". Hope you find this useful. Have a great day :)

No tengo idea tampoco Alguien nos explicaría ?