In the beginning of the novel, Jonas struggles to accurately describe how he feels about the Ceremony of Twelve, and finally decides that he feels apprehensive.
After his training with the Giver is underway, it does not take him long to realize that the accuracy he worked so hard for before was no longer sufficient. He realizes that the sharing of feelings at the dinner table each night is empty and void of true emotion. After all, through the memories, he has felt genuine fear, love, anger, etc.
Once he understands that the war games his friends play aren't just a game, he tries to express sadness, and anger to discourage them from playing. Of course, this does not work because his friends do not have the depth of emotion that Jonas now has.
The most noticeable expression of his feelings is when he learns that Gabriel is scheduled to be released the next morning. Besides just expressing his horror with words and tears, he shows his emotion with action. Since he loves Gabriel and considers him to be family, he feels he is left with no alternative but to escape and save him.
At first he expresses himself verbally, which is the only option available to him. Once his training is underway and he has received memories, his thoughts begin to change, which moves his level of expression from words to action.