At what points in the story does Dickens change from the past tense to the present tense and from third-person to first-person narration?in a Tale...

Perhaps the most salient example in A Tale of
Two Cities of Dickens's authorial intrusion comes in Chapter 3, "The Night
Shadows" of Book the First:

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A wonderful fact to reflect upon, that every
human creature is constituted to be that profound secret and mystery to every other.  A
solemn consideration, when I enter a great city by night, that every one of those darkly
clustered houses encloses its own secret; that every room in every beating heart in the
hundreds of thousands of breasts there, is, in some of its imaginings, a secret to the
heart nearest it!  Something of the awfulness, even of Death itself, is referable to
this.  No more can I turn the leaves of this dear book that i loved, and vainly hope in
time to read it all....In any of the burial-places of this city through which I pass, is
there a sleeper more inscrutable than its busy inhabitants are, in their innermost
personality, to  me, or than I am to
them?

In the opening chapter
of his novel, Dickens draws parallels between historical periods and the present period;
here again in this chapter, Dickens wishes the reader to understand that he has observed
in his time what others have noticed in the past.  In this way, the reader takes notice
of the universality of his themes.

In addition, the tense
change to present creates an immediacy to a passage such as that at the end of Chapter
21:

Seven
prisoners released, seven gory head on pikes,...--such, and such-like the loudly echoing
footsteps of Saint Antoine escort through the Paris streets in mid-July, one thousand
seven hundred and eighty-nine.  Now Heaven defeat the fancy of Lucie Darnay, and keep
these feet far out of her life!  For, they are headlong, made and dangerous; and in the
years so long after the breaking of the cask at Defarge's wine-shop door, they are not
easily purified when once stained
red.

Again Dickens ties his
narrative and its historical context to his time as, influenced strongly by Thomas
Carlysle's The French Revolution:  A History in which the author's
idea that history cycles through destruction and resurrection was an important influence
on the novel.