Tense is the grammatical expression of the location of events in time. It anchors (or ‘grounds’) an event to the speaker’s experience of the world by relating the event time to a point of reference. The normal, universal and therefore unmarked point of reference is the moment of speaking – speech time, what has been called ‘the inescapable and constantly changing now in which all verbal interaction takes place’. Past event takes place before the ‘now’, while future events are thought of as taking place after it. (Angela Downing, English Grammar: A University Course)
I don’t grasp what she says about the tense exactly. So I brought an example below. In the example, it seems that the ‘speech time’ is past, for the whole story is mainly developed by the tense. Having thought this, I’m confused at the ‘present’ dialogues: “I think you must. . .”, “See? Harry Potter. . .”
Would you let me know what is ‘speech time’, and why it can be constantly changing now?
"Hagrid," he said quietly, "I think you must have made a mistake. I don't think I can be a wizard."
To his surprise, Hagrid chuckled.
"Not a wizard, eh? Never made things happen when you was scared or angry?"
Harry looked into the fire. Now he came to think about it... every odd thing that had ever made his aunt and uncle furious with him had happened when he, Harry, had been upset or angry... chased by Dudley's gang, he had somehow found himself out of their reach... dreading going to school with that ridiculous haircut, he'd managed to make it grow back... and the very last time Dudley had hit him, hadn't he got his revenge, without even realizing he was doing it? Hadn't he set a boa constrictor on him?
Harry looked back at Hagrid, smiling, and saw that Hagrid was positively beaming at him.
"See?" said Hagrid. "Harry Potter, not a wizard - you wait, you'll be right famous at Hogwarts." (Harry Potter and the Sorcerer's Stone)