Sometimes they don't have a strong grasp of English because they aren't native speakers. Sometimes they have learning disabilities OR an intellectual disability/below average intelligence. They could forget basic rules if they don't write that often. They could have had a stroke, brain injury, or dementia. They could have had a lousy education for whatever reason.
There are lots of possibilities. I try not to judge people for it. There was a time when I struggled with writing a lot, too. My education, as a homeschooler, was really bad.
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“Tú, que me lees, ¿estás seguro de entender mi lenguaje?” — Jorge Luis Borges