What does Annie mean when she says, "I'll tell you what I pity, that the sun won't rise and set for all her life and every day you're telling her it will. What good will your pity do her when you are under the strawberries, Captain Keller?"

This passage happens in act two, soon after Annie's arrival in the house. While at a meal, Helen continues to reach onto each person's plate and take whatever food she wishes. Normally, she gets away with this behavior without consequence:

Kate: Miss Annie. You see, she's accustomed to helping herself from our plates to anything she—
Annie: [evenly]: Yes, but I'm not accustomed to it.

At first the family thinks that Annie is upset that her food is being touched by the child. Mr. Keller calls to Viney, their servant, for a fresh plate. However, this is not Annie's ultimate concern; her primary concern is Helen's power to manipulate her family and bend them to her wishes. In other words, she is primary concerned by how spoiled and stubborn Helen is. The family continues to try to get Helen a new plate. Kate, Helen's mother, explains:

Kate: . . . I'm afraid what Captain Keller says is only too true, she'll persist in this until she gets her own way.

Annie is not willing to accept this behavior. She cannot understand why the whole family is so willing to let Helen have her way, feeling around their plates for food and eating without any table manners. They soon reveal that they allow her this freedom because they pity her. Mr. Keller accuses Annie of lacking pity:

Keller [very annoyed]: Miss Sullivan! You would have more understanding of your pupil if you had some pity in you! . . .
Annie: Pity? For this tyrant? The whole house turns on her whims, is there anything she wants she doesn't get? I'll tell you what I pity, that the sun won't rise and set for her all her life, and every day you're telling her it will. What good will your pity do her when you're under the strawberries, Captain Keller?

In this quote Annie is reminding Mr. Keller (and his wife) that not everyone is going to let Helen act however she wishes. Annie reminds the family that their lack of rules for Helen has allowed her to become a "tyrant" who uses power to create chaos and stress in their household. Though she gets whatever she wants from them, Helen can do little for herself. They have not prepared her for life outside of their house. Annie reminds Captain Keller that he will not live forever and that others will one day be responsible for Annie. (This is what she means when she reminds him that he'll one day be "under the strawberries"—that is, in his grave.)
Though she gets whatever she wants, Helen can do little for herself because no one believes she is capable. She has not been taught any useful skills because of her physical disabilities. This passage infers that Annie would like to teach Helen and to give her tools to learn and live more independently in the future. Annie believes Helen is far more capable of learning than her family does. Right now, Helen is limited by her family's pity. For example, the family does not think that Helen can ever learn table manners because she is blind and deaf. Annie defies this belief and immediately insists that Helen learn the same table manners that the rest of their society uses. Ultimately, the family's pity toward Helen limits her potential. Annie recognizes the potential that Helen has to learn and behave like anyone else.