It is hard to pinpoint exactly when Elizabeth learned the truth behind Ann Boleyn's death, but accounts from the time suggest that, although she was never officially told, she figured it out for herself. The beheading of Catherine Howard, when Elizabeth was eight, perhaps traumatized her even more than Ann's death. It was at this time that she promised herself (and started telling her friends) that she would never marry. No one took her seriously at the time, but this was a decision she stuck to throughout her life.
After Ann's death, Elizabeth was marginalized for a long time, sent to one of the unimportant royal properties, where she received hardly any new clothes at all. This deprivation as a child may explain Elizabeth's adult obsession with fine clothing and jewelry, as well as her fiscal prudence.
Elizabeth's intelligence was always apparent to her tutors. At the time, this was unexpected in a girl. Elizabeth quickly attained fluency in Latin, French, Spanish, and Italian, and Flemish, knew some Greek, and even secretly learned Welsh. She spent her time studying the history and the classics under renowned scholars, all of whom were charmed by the precocious princess. Edward and Elizabeth wrote their letters to each other in Latin.
Henry's will outlined the following succession order: (1) Edward (2) Mary (3) Elizabeth (4) the family of Henry's sister. This will excluded the Stuarts completely. But Catholics had difficulty arguing with Elizabeth's succession: although she was considered a bastard (Elizabeth was conceived prior to Henry's marriage to her mother, and then the marriage took place while Henry was still technically married to his first wife), her succession was in the King's will. Moreover, Elizabeth had clever and skillful allies: even at this young age, Elizabeth already was becoming close with William Cecil, who would be a trusted advisor throughout her life.
Before Henry's death, the younger, prettier, more charming Elizabeth always enjoyed more popularity with the people than did Mary. Besides having a more magnetic personality than her droopy half-sister Mary, Elizabeth had the added advantage of being a Protestant (Mary was a Catholic) and resembling her father, Henry VIII, in hair-color, bearing, face, and eye-color. For this and other reasons, Mary hated her sister: Henry had his marriage to Mary's mother, Catherine of Aragon, annulled so that he could marry Elizabeth's mother Ann Boleyn. Elizabeth had even been conceived adulterously, before Ann and Henry were married, and while Catherine and Henry were still officially married. As a Catholic, Mary considered Henry's marriage with Ann to be illegal, and she therefore always considered Elizabeth to be an illegitimate daughter of Henry. When Mary discovered the plot against her and suspected Elizabeth of involvement, she was quick to imprison her. However, Elizabeth's quick wits and capacity to love her enemies kept her alive during her internment in the Tower: by providing only sly "answerless answers" to her interrogators, Elizabeth made it impossible for anyone to prove her guilt in the plot to overthrow Mary; at the same time, she wrote her half-sister heart-rending letters. Thus the young princess manipulated her half-sister's emotions so well that Mary could not bring herself to order a beheading.
Elizabeth's Protestantism was yet another threat to Mary. Yet while Mary was a devout Catholic, Elizabeth was only a moderate Protestant, who really wouldn't have minded all the splendor of the Catholic Church. Even as a child, she showed her sense of religious tolerance by saying that the religious disputes of the Reformation were over mere "trifles." Nonetheless, Elizabeth had the sense to know that appearing Protestant made her popular with the English population, then mostly Protestant.