托福备考句中的核心主干成分同学们理解为一个大的家庭：爸爸、妈妈、孩子、姥姥、姥爷、爷爷、奶奶，七大姑八大姨等等，常规意义上来讲构成一个小家庭的单元是要有爸爸和妈妈，同时还有孩子。
那么，我们可以理解为三大主干成分：爸爸是主语，妈妈是谓语，孩子是宾语或者表语，谓语也就是妈妈负责连系子女与父亲，在句子中及家庭中起道尤其重要的作用（灵魂作用）。其他家庭成员为修饰成分，让这个家庭变得更加幸福和谐。
接下来，我们从爸爸的小时候讲起，大家端起小板凳，耐心来听老师讲解。一般主语表示句子的谈论对象或者动作发出者，通常具有以下特点：
1）由名词（短语）、代词、不定式、数词及句子等来充当
2）位置在句首居多，位于谓语动词之前
3）数量可以一个也可以多个（非简单句中）。
通过托福指南（OG）的部分句子，老师带大家来认识一下主语：
Desertification is accomplished primarily through the loss of stabilizing natural vegetation and the subsequent accelerated erosion of the soil by wind and water.
Desertification（名词）
The semiarid lands ordering the deserts exist in a delicate ecological balance and are limited in their potential to adjust to increased environmental pressures.
主语：
The semiarid lands（名词短语）
He refused to develop projection technology, reasoning that if he made and sold projectors, then exhibitors would purchase only one machine-a projector-from him instead of several.
主语：
He（代词）；he（代词）; exhibitors（名词）
Cheap transportation networks, the rise of cities, and the availability of capital and credit all stimulated the shift to factory production.

主语：
并列三个Cheap transportation networks，the rise of cities，the availability of capital and credit（均为名词短语）
That this device was a necessary structural compromise is clear from the fact that the cannonball quickly disappeared when sculptors learned how to strengthen the internal structure of a statue with iron braces (iron being much stronger than bronze).
That this device was a necessary structural compromise...（句子）
This device(名词）
the cannonball（名词）
Sculptors（名词）
以上老师给了一些常见几种主语形态，同学们要继续补充比如数词，不定式等其他形式充当主语的情况喔。