Think about how you learn to read - you 'sound out' words, turning letters into sounds to match them to a pronunciation to figure out what word is represented.

Think about how you learn to read - you 'sound out' words, turning letters into sounds to match them to a pronunciation to figure out what word is represented. Now imagine trying to learn how to do that when you have never heard any words spoken out loud. People who are deaf from birth often have a lot of difficulty with spelling and reading, because both skills are closely connected to saying and hearing words. Connecting written words to lip movements (which is kind of the closest thing to 'phonics' for a deaf person) is lossy - the letter-to-lip connections are fuzzier than letter-to-sound, and lip-to-letter is very ambiguous. Subtitles are great for people who are confident and comfortable readers - say, people who have become deaf due to age - but for some deaf people following subtitles can be like asking someone who's dyslexic to quickly read a sentence out loud.