Your comment (October 14) is right: "Most common errors revolve around homophones (theretheir), the doubling of letters (arisearrive), and words in which the vowel sounds in the final syllable is unclear (pottervisitor)."
These errors are not individual, but common to all learners who have difficulties with spelling. They stem from the fact that the spellings of homophones, doubled letters and word endings and some prefixes (decidedivide) don't follow logical rules and so have to be individually memorised.
Anyone with a poor visual memory, irrespective of their general intelligence, has trouble imprinting those words on their memory and so finds learning to spell very hard. My son, who attended the local comprehensive and then went to Cambridge, was one of them, having inherited this handicap from his father.
Just explaining this fact to poor spellers can boost their self-esteem and help them to cope better. What is unhelpful to them is pretending that there is either hidden logic or cultural value behind English spelling inconsistencies.
A few indicate defunct Latin prefixes. We have, for example, "pp" in "apply" because it comes from the Latin "adplicare". But most of our quirky spellings are simply typesetting errors or whims of foreign printers who could not speak English.
This happened mainly because William Tyndale, who produced the first modern English Bible, had to flee abroad (Sir Thomas More was after his head for wanting to enable every ploughboy to read the word of God for himself, rather than have to rely on what a priest interpreted for him).
Tyndale's New Testament was the first English book to sell in large volume.
People were exceptionally keen to read it. They learnt to spell from it too, not knowing that its many editions with their numerous spelling differences, were full of errors.
If we made more children aware of this, they might stop feeling so bad about not being able to spell and stop being distracted by this disability in their writing. Steinbeck did. The only danger is that they may start questioning the way we spell and even refuse to spell so many words as stupidly as we continue to do ("many" being one of them, "one" another).
Masha Bell Author of Understanding English Spelling