The Dursleys had never bothered with reading to Harry or playing word games with him as a young child, so when he started at muggle school he needed extra help. One of the teaching assistants had sat with him every day in his first year of school to catch him up. He had managed to pick up enough to get by but it was not until he got to Hogwarts and met Hermione he really learned how to spell properly and structure a sentence. If it had not been for Hermione Harry knew he would probably still struggle with basic English.