De plus definite article

I am confused as to when you have to include the definite article, contracted with ‘de’. For example, why is it “A paragem de autocarro” rather than “A paragem do autocarro”?

Again, why is it “Nós sentámo-nos debaixo de uma árvore” rather than “debaixo duma árvore”?

@pvreed, about “paragem de autocarro” vs “paragem do autocarro”, I can tell you that the first is more general (just how you’d typically call any bus stop) and the second is more specific (how you’d refer to a bus stop for a specific bus route). For example:

  • O horário está disponível em qualquer paragem de autocarro - The timetable is available in any bus stop
  • Estás na paragem do autocarro 149? - Are you at the stop for bus 149?

As for de uma vs duma, that’s an optional contraction, so you might see it either way for no particular reason!