why was jonas so careful in his usage if language

Jonas has to be careful with his use of language because the community has certain expectations and requirements for the language is used. These requirements are all a part of the concept of "sameness."

An example of this can be found at the start of the book when Jonas deliberates over his own use of the word frightened. He believes himself to be frightened, but then second guesses the words meaning, and the way it pertains to his own feelings. Language comes down to the use of appropriate words and stewing over it when you believe you've said something wrong.