It is unclear as to what exactly causes dyslexia.
We do know that dyslexia is a learning disorder that is language-based. For those that have this disorder, it is likely that they will have a difficult time with reading. They may also experience other problems with language skills: pronouncing words, spelling, and writing.
The majority of people, whether diagnosed or undiagnosed with dyslexia, have problems with speech sounds in words and have problems learning how each letters represents those sounds.
Spoken language can also be difficult for people that have dyslexia even after proper exposure to proper language being used in school or their homes. It is likely that they will have difficulties expressing themselves clearly and they may not fully comprehend the meaning of what others say.
Researchers have studied the reading skills of children with specific language impairment (SLI) and found literacy problems in children who showed language impairment at an early age. Source: http://www.thepowerofdyslexia.com/
The underlying cause of phonological deficits in poor readers is unclear. One possible source is impaired perception of speech at the phoneme level. Several studies have shown that poor readers as a group tend to perform abnormally on tasks involving the categorization and/or discrimination of speech sounds (Godfrey, Syrdal-Lasky, Millay, & Knox, 1981; Manis et al., 1997; Masterson, Hazan, & Wijatilake, 1995; Mody, Studdert-Kennedy, & Brady, 1997; Reed, 1989; Werker & Tees, 1987).
This learning disability makes it very difficult for students to be successful in a regular school environment. We know that if teaching methods are appropriate, for example explicit, systematic phonics are taught then people with dyslexia can successfully learn how to overcome reading difficulties.