Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Various
Shmoop doesn't mean to be elusive here, but we're talking about one hundred stories and eleven different narrators (counting the frame story). So to be clearer, have a look at this breakdown:
Frame Story/Narratorial Intrusions
First Person (Central Narrator)
In the "Prologue," at the opening of Day 4, and in the "Author's Epilogue," we hear the author speak of his reasons for writing this work. He defends himself against the jealousy of his contemporaries and explains his love for the ladies and his own romantic disappointments.
The Novelle
First person (peripheral and central); 3rd person (omniscient)
So we have a narrator setting up the narrative situation for us, describing the situation of the brigata. Then we're given ten other narrators, who in turn each tell ten stories with characters who are sometimes telling stories themselves. If you think about it too long, we guarantee your brain will hurt. Let's break it down further:
You already know that the author opens each day with his narration of the storytellers going through the daily routine and getting ready to tell their stories (third person omniscient).
He also provides the transitional bits at the top of each story, which look like this: "[...] Lauretta, who was seated next to [Pampinea], took up her cue without a pause and began to speak as follows [...]" (II.4.91). Generally speaking, this kind of narration happens in third person omniscient voice.
Our young storytellers often open their stories with first person introductions and directly address their audience before narrating in the third person omniscient voice. In the passage following the one we've given you above, you can see how this works:
'[...] When I think of the previous story, [I know] that my own will be followed less attentively. But since it is the best I can manage, I trust that I shall be forgiven. Few parts of Italy, if any, are reckoned to be more delightful than the sea-coast between Reggio and Gaeta [...]' (II.4.91-92).
Holy mucca! Within a few paragraphs, we've just heard from the narrator (third person omniscient) and Lauretta (both first person central and third person omniscient). To make matters even more interesting, the characters in the stories often tell stories themselves. Boccaccio does this kind of thing throughout, so you've got to pay attention.