History of Henry Esmond, Esquire Book

It would be hard to find a more virtuous hero than Henry Esmond: finding out that he's not the bastard child he thought he was, but in fact the true heir of a title and fortune, he declines to claim his title and property, leaving it to a sillier, younger man. Or is Henry the less silly man? He moons and mopes over the beautiful Beatrix (his foster-sister) for ten years, declining to see her assholish, power-hungry ways, until he is finally so put off by her flirtations with the horny James Stuart, the royal Pretender, that he decides instead to marry her mother (who was his own foster mother), now in her "autumn" years. (Well, yes.) Though old-ish, his new wife is "as pure as virgins in their spring" (whatever!) and bears him a child. They move to a Virginia plantation and he sells the diamonds he had once given Beatrix to buy "negroes" who are "the happiest and merriest, I think, in all this country..." Oof.

I'm leaving a lot out. Henry goes to Cambridge, finds out who his mother is, finds out who his stepmother is, engages in much Tory politicking and Jacobitism, makes friends with Richard Steele and Joseph Addison, fights in the War of the Spanish Succession, spends time in prison for his part in a duel, annoys Jonathan Swift, does a lot of switching between Protestantism and Catholicism, writes a play that fails, and hatches a plot to restore James Stuart (who would be James III) to the throne.

I was perplexed by the constant switching between past and present tense, and would like an explanation from Thackeray.