章振邦先生在新编英语语法教程的简单句与复杂句中讲到：

如果构成分句成分的词组本身带有从属分句，那么这个句子仍然是简单句。例如：

The students I teach have made better grades in the past few weeks.请问这个词组本身带有的从属分句是红字部分吗？为什么词组带有从属分句就是简单句

但若某个句子成分直接由从属分句表示，那么这种句子便不是简单句而是复杂句。例：

The students would have made better grades if they had studied hard.

The boy can stay in the room so long as he keeps quiet.

What he said is not true.   I didn't understand what he meant.  请问老师直接由从属分句表示是什么意思？这四个句子与第一个有什么区别？还请老师明晰讲解简单句与复杂句的区别。感谢！