给出英文文章的一段内容，将其中的语法错误和拼写错误全部改正，并解释更改的原因。
"John's been studying maths for over a year now, and his knowledge of the subject have considerably increased".
"John has been studying math for over a year now, and his knowledge of the subject has considerably increased."

改正原因：
1. "Math"应该用单数形式，因为它是一个不可数名词。
2. "Has"应该用来和主语"knowledge"保持一致，即用单数形式。
3. "Considerably"是正确的拼写，没有错误。