I'd say it's the same difference between nice and polite.

I agree with Joel, you can also say: ¿De qué tamaño quiere la torta? In the Southern Cone, cake is "torta" and in the rest of Latin America, cake is "pastel", I believe.

It's /biba la bida/

The verbs "parar" and "detener" can mean "to stop", yes. But sometimes you cannot replace one with the other. For instance: A. Stop threatening me! = (1) ¡Pará con las amenazas! ≠ (2) Detente con las amenazas. Here senteces A1 and A2 differ in their collocations. The verb "detener" does not allow the pattern "con" + direct object. A correct equivalent would be "¡Deja de amenazarme!" B. He was stopped by the police. = (1) Lo paró la policía. ≠ (2) Lo detuvo la policía. Here sentences B1 and B2 mean different things. In B1, he was just stopped for, let's say, breaking the speed limit. He may have got away with just a ticket. But in B2, he was arrested! Hope it helps!

Tenés que ir al dentista antes de que pase la semana.

Me gustaría hablar español para conseguir un mejor trabajo y ganar un buen sueldo y también para hablar con los mejicanos.

That is because in Spanish "day" is thought as a process or experience and not as an entity. To illustrate, let me give you a couple of examples: A. How is she? B. How is the soccer game going? C. How did the surgery go? In sentence A, "she" is referring to an entity, a person. In sentence B, "the soccer game" is seen as an activity that unfolds through time. In sentence C, "the surgery" is seen as a surgical procedure. Spanish speakers think about "day" as English speakers think about "game" and "surgery". Thus, the verb "go" in this examples is rendered as "ir" or "va" when conjugated in simple present 3rd person singular. As for why do we think about "day" like that, language is arbitrary. We do it just because, there's no reason behind it.