1999年考研英语（旧卷）真题句子解析： But this was not always the case; before it was legally required for all children to attend school until a certain age, it was widely accepted that some were just not equipped by nature to pursue this kind of education.

But this was not always the case; before it was legally required for all children to attend school until a certain age, it was widely accepted that some were just not equipped by nature to pursue this kind of education.

分号连接两个独立的主谓结构：第一个分句是"this was not always the case"，第二个分句是"it was widely accepted that..."（含时间状语从句"before..."）。两个分句都有完整的主谓宾结构，通过分号对等连接，形成并列关系。语义上，前句否定过去一直如此，后句解释过去的实际情况，构成转折对比关系。句首"But"强化了转折语义。

before it was legally required for all children to attend school until a certain age, it was widely accepted that some were just not equipped by nature to pursue this kind of education

第二个并列分句包含两层嵌套：第一层是时间状语从句"before it was legally required..."修饰主句"it was widely accepted..."；第二层是主句中的主语从句"that some were just not equipped..."。同时，时间状语从句内部也包含不定式复合结构"for all children to attend school"。这形成了时间从句与主语从句的双层嵌套结构，加上不定式的嵌入，整体构成典型的从句套从句多重嵌套。

it was legally required for all children to attend school

"it"是形式主语，真正的主语是后置的不定式复合结构"for all children to attend school until a certain age"。这是典型的"it + be + 过去分词 + for sb + to do"句型，形式主语"it"占据主语位置，避免头重脚轻，真正的主语（不定式复合结构）后置。去掉"it was...required"后，"for all children to attend school"是真正被要求的内容。

to attend school

"to attend"是不定式，在"it was legally required for all children to attend school"结构中作主语补足语。这是"require sb to do"被动形式的变体，真正的逻辑主语是"all children"，不定式"to attend school"说明被要求的内容，补充说明主语需要做什么。

equipped by nature

"equipped"是过去分词，在被动结构"were just not equipped by nature"中作主语补足语，表示被动含义。主语"some"（指代某些孩子）与"equip"（装备、使具备能力）之间是被动关系——孩子们被天性赋予能力，而非主动装备自己。这里"equipped"补充说明主语"some"的状态，属于典型的过去分词作主语补足语用法。

to pursue this kind of education

"to pursue"是不定式短语，作后置定语修饰"equipped"的隐含宾语。完整理解是"equipped (with ability) to pursue this kind of education"——具备追求这种教育的能力。不定式表示能力或资格的方向，说明"equipped"所指向的具体内容，是典型的不定式作后置定语用法。

it was legally required for all children to attend school

本句中 'it was legally required for all children to attend school' 是典型的形式主语结构。

结构拆解：'it' 是形式主语，无实义指代；'was required' 是系动词 be 的过去式 + 过去分词形容词化形式；'for all children' 标明不定式的逻辑主语；'to attend school until a certain age' 是真正主语（不定式短语）。

还原验证：去掉形式主语 'it was'，真正主语前置后句子变为 'For all children to attend school until a certain age was legally required'，结构完整但头重脚轻，因此用 it 作形式主语将真正主语后置。

'for + 名词/代词' 的作用：当不定式有自己的逻辑主语（执行动作的人）时，用 'for sb to do' 结构明确指出。本句中 'attend school' 的执行者是 'all children'，故用 'for all children'。

识别要点：(1) it 不指代具体事物；(2) be 动词后接形容词性成分（过去分词 required 作表语）；(3) for sb 标明逻辑主语；(4) to do 不定式是真正主语。

迁移方法：遇到 'it + be + 形容词/过去分词 + for sb + to do' 时，先判断 it 是否有实际指代；若无，则识别为形式主语结构，真正主语是后面的不定式，for sb 表明动作的执行者。