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.
At the beginning of the novel, Jonas is what his society has produced. The society in which Jonas was living was very closely monitored and regulated. Families were not meant to be close and communicative. Natural urges such as physical desires and the pursuit of personal choice and dreams were very tightly controlled. Individual goals and motivations were meant to take a back seat to what the government established was “best” for the community, and the government very closely watched individual behavior. It is not surprising then that Jonas would be so careful in his use of language. The only time Jonas ever really deviates from this is through his interactions with the Giver when they are perhaps not so closely monitored, and at this point Jonas is learning that there are alternatives to the behaviors and ideas he has been raised to have.