What did Jonas learn about the community's rule about lying, in chapters 16 to 19 of The Giver?
Jonas already knows before this that he has been given permission to lie as part of his job description as Receiver. When he learns this, he wonders if other people in the community also have that same permission, but he has no proof one way or another.
During these chapters, Jonas's father mentions that a newchild will be released soon, because a mother is expecting twin boys. It is the policy of the community not to allow identical twins to live there, since it might confuse people if they lived with different families (which they would). Jonas asks his father if someone from Elsewhere will come and get the child, and his father tells him that that is how it happens; he will choose the child to be released and someone from Elsewhere will come and get him.
The day of the release, Jonas asks the Giver about it, and the Giver pulls up a video of Jonas's father releasing the twin boy. Jonas discovers that release really means euthanasia, and that his father has killed the baby and disposed of the body instead of sending the baby to live Elsewhere. Jonas is extremely upset, realizing that this means his friend Fiona is killing the Old on their release days as well, and no one in the community knows the truth. This means that people who deal with release are lying to the other members of the community; he wonders who else is lying, and about what.