People with Down Syndrome usually know that they are different than others and are often teased or harrassed and called retarded. Although they are retarded it has negative connotations which hurt their feelings and reinforce feelings of low self-esteem. I hate it when someone uses the word "retard" to insult another person. This is also hurtful.
Individuals with Down Syndrome usually have some speech impairments which is frustrationg for the individual and often causes them to give up trying to communitcate or think that they are bad for being unable to speak properly. With my clients, I used to tell them that "it is my job to try to understand you, but I may need you to slow down or use other words. I would also ask them to be patient with themselves as it is important for me to hear what you want to say." It takes patience and you get better at it as you become more familiar with the person, just as it is with other people that you meet.