Why was Jonas so careful in his use of language?In Lois Lowry's story 'The Giver.'

This first answer explains to what extent the individual is controlled in the Community. However, the fact that Jonas is so careful about his speech is not really something unique which distinguishes him from others; rather, he has been conditioned to choose his words carefully since this is one thing highly valued by the Community itself. At the Ceremony of Twelve, for example, when it was (Jonas' best friend) Asher's turn to receive his Assignment, the Chief Elder related an amusing anecdote about Asher receiving a "smack" instead of a snack until he could pronounce the word properly.

Controlling speech is indeed a way to control thoughts and feelings, so it is a shock to Jonas to receive the permission to lie in his role as the Community's new Receiver of Memory. He begins to wonder if everybody else has received the permission to lie as well and starts to doubt the credibility of the Communitiy's values system as a whole. In a way this prepares him for another shock to come when he understands the true meaning of being "released," and it spurs him on to his final decision to escape with Gabriel before he, too, is killed by simple protocol.