Please can you explain the difference between 'por' and 'para'?

I think it is best to answer this question by looking at some examples  1. Llegué temprano a la cena por hablar a Juan   I arrived early to the dinner due to speaking to Juan 2. Llegué temprano a la cena para hablar a Juan. I arrived early to the dinner (in order) to speak to Juan  As you can see just by changing the word from por to para the sentences have different meanings. Por is related to cause, the reason why something happened: if you asked the sentence in a question the answer would start with 'because', 'why did you arrive early?' 'because i was speaking to Juan', whilst as para is related to the aim, the end goal of the action, here the answer starts with 'in order to': 'why did you arrive early?' 'in order to speak to Juan'.  I find this subtle difference the hardest, however there are other cases where there is a concrete rule for example TIME:in the following cases you must always use 'por': for the time of day: Estudio por la noche : I work at night / for frequency: tres veces por semana: three times a week / for duration: trabajo por una hora: i work for an hour (here can be replaced by 'durante') PLACE: similarly in these cases involving location you must always use por:   for a rough location: creo que vive por aquí: I think he lives around here /  for saying through something: entró por la ventana: It entered through the window / for saying along something: paseaba por la playa: I walked along the beach. There are a few other cases however I think remembering the above is a very good start.