Tom was more serious than he had been.（划线部分是形容词比较级，不能改为 much serious）

    I'm not much good at this sort of game,but I'll give it a go.

    此句中的 much good 不能改为more good：在形容词或者副词原级前面用 much 表示“非常”、“很”的意思，只限于某些特定情况，如：He was much different from many boys. 他与别的普通男孩很不一样。/ He is not much good at writing essays. 他不太善于写文章。/ I woke up (very) much early in the morning. 早上我很早就醒了。（摘自《外研社英汉多功能词典》p.1376）