主语从句是指在英语中用作主语的从句。若从句修饰主语，则它可能属于多种从句类型。比如，当主语是名词或代词，修饰主语的从句通常被称为定语从句。如：

“All the apples that fall are eaten by the pigs.” 这里，that fall修饰主语all the apples，是定语从句。

“A man who sells books in a shop is called a bookseller.” 其中，who sells books in a shop修饰主语a man，是定语从句。

如果主语是动词的某种形式，如动名词或不定式，则修饰主语中动词的从句可能为状语从句。例如：

“To have a good sleep when you are very tired is a sheer paradise.” 这里，when you are very tired 修饰主语to have a good sleep，是状语从句。

“Wanting to get well again when you're ill is often half the battle.” 其中，when you're ill 修饰主语wanting to get well again，是状语从句。

当然，对于后两句，更常见的表达方式可能是：

“When you are very tired, to have a good sleep is a sheer paradise.”

“When you're ill, wanting to get well again is often half the battle.”