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Enunciate well, and focus on clarity. Speaking louder doesn't make you more understandable. Consider this an opportunity to work on your clarity of speech. If they say the word "gigantic," then they probably also know what "enormous" and "huge" mean. If they speak using basic words, then it's probably best to use the smallest words you know. If they use words like "fortuitously" and "systematic bias," then their disability probably isn't intellectual. If the person seems to struggle understanding speech, keep your sentences short and clear. Use simple subject-verb-object statements when you can. This is good practice in general too. Non-disabled people don't enjoy wading through extremely long sentences either. If the person is hard of hearing or struggles to process speech, they may want to watch you as you pronounce your words. This helps them figure out what you are saying in many cases. Avoid turning away as you speak, covering your mouth, or speaking with your mouth full. It can also be helpful to talk in quieter places, with fewer distractions, especially if the person seems bothered by environmental noises. For example, the question "Do-ya wanna eat-a pizza?" may be difficult for them to understand. One of the biggest challenges for listeners is knowing where one word ends and the next one begins. If they seem to be struggling, slow down the pace a little, giving a slight pause between each word. There's no need to use baby talk, or mimic their disability accent. (No, it won't help them understand you better, but it may make them think you're mocking them.) Talk to them with the same tone that you'd use for a non-disabled person their age. Baby talk may be appropriate for a disabled 3-year-old, but not a disabled 13-year-old or 33-year-old.
Speak calmly, clearly, and with a moderate volume. Model your vocabulary usage after theirs. Keep your sentences short and clear, if needed. Let them see your mouth if they can't understand you well. Avoid running words together if it confuses them. Use your normal pitch and tone.