A lot of people on the spectrum who I am acquainted with have difficulties with spelling/grammar.
The most common things I have noticed people on the spectrum struggle with are:
* The correct conjugation of words
* The correct use of pronouns
* Using the right words in the right context
* Stylistic difficulties, such as run-on sentences, punctuation, duplicated words in a sentence, etc.
Also, difficulties with syntax in general.
For some reason, I've always had difficulties spelling words that contain double consonants (i.e. Mississauga, accommodation). It took me two years to deliberately teach myself the spelling of the word Mississauga, and I've only grasped the spelling of the word accommodation several months ago. I always used to spell it with one "m". I also used to write horribly long run-on sentences, and on that I had to learn to catch myself doing it every time. I still duplicate words in sentences a lot.
It's probably those who have a specific learning disability who end up having difficulties with spelling and grammar. Since the brain of every person with AS/autism is unique, the way the brain ended up being configured in some people on the spectrum, Broca's area, which is responsible for the sequencing of speech sounds, or the right angular gyrus which is responsible for the non-verbal perception of the word, for example, may have been affected for that person in a certain way that has led to impairment in grammar/spelling.
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Leading a double life and loving it (but exhausted).
Likely ADHD instead of what I've been diagnosed with before.