She stood there motionless, her eyes filled with tears—whether of shame, frustration, or grief it was difficult to tell.  

motionless 是形容词，为何修饰动词sat? 原句中的用法是固定搭配吗？

再比如 drive safe, 而不是drive safely. 

drive safely 属于动词drive+副词safely（作状语）的用法

解答如下：

以上有以下2种理解

(1)形容词作方式状语

motionless adj.  不动的，静止的; 呆若木鸡的; 不运动的)

(2)属于双重谓语用法

She stood there motionless

=She was motionless when she stood there.

至于用哪种方式去理解并掌握以上语言现象不重要，重要的是，你需需要熟练使用某种方式去理解以上语言现象。

(1)形容词作方式/伴随状语

The girl kept crying cold and hungry.

They went home tired and hungry

He spent 7 days in the wind and snow，cold and hungry．

They all rushed up，eager to help．
She returned to work，unhappy．
I dont like to sit here doing nothing．

(2)双重谓语用法

双重谓语的基本形式是：行为动词+表语（说明主语从事该行为时所处的状态），用作双重谓语的动词多为那些表示位置移动变化的行为动词。如：return，stand ,leave， go，come，arrive，start等。其具体的结构形式主要有以下几种：

1．动词+形容词

（1）He left this morning very gay．（=He was very gay when he left this morning．）上午离开时，他很开心。

（2）She lay there motionless．（=She was motionless when she lay there．）她躺在那儿一动不动。

2．动词+名词

（1）She left a shy girl and returned a young mother．（=She was a shy girl when she left and a young mother when she returned．）她离开时还是一个害羞的女孩，回来时却是一位年轻的母亲。

（2）He lived a hero and died a martyr．（=He was a hero when he lived and a martyr when he died．）他生前是英雄，死后为烈士。

3．动词+介词短语

（1）He died in despair．（=He was in despair when he died．）他死时非常绝望。

（2）He left in tears．（=He was in tears when he left．）他离开时眼含泪水。

（3）He ran into the room out of breath．（=He was out of breath when he ran into the room．）他冲进房间时已是上气不接下气。

4．动词+分词（短语）

（1）They lay there chatting．（=They were chatting when they lay there．）他们躺在那里聊天。

（2）He wandered wondering．（=He was wondering when he wandered．）漫步时，他在冥思。

（3）He came home exhausted．（=He was exhausted when he came home．）他回家时已是筋疲力尽。

5．动词+what从句

（1）He returned home what he had always been．（= He was what he had always been when he returned home．）回家时，他还是老样子。

双重谓语结构很少有否定形式，但偶尔也出现否定形式。这时，要特别注意否定的形式在前，意义在后，即否定后移现象。如：

（1）She didn't arrive accompanied by her daughter．（=She was not accompanied by her daughter when she arrived．）她抵达时不是由她的女儿陪同。

（2）He didn't pass noticed．（=When he passed he was not noticed．）他路过时没人注意。

特别备注:

只要你能识别以上语言现象，并能熟练使用以上语言现象，你用以下哪种方式理解都可以.遇到语法观点有分歧，就选一种你最能接受那种观点去理解，去使用，语言最重要是能自己快速识别以上语言现象并熟练使用这一语言现象就OK 啦。

(1)双重谓语来理解以上语言现象

(2)形容词作方式/伴随状语来理解

(3)形容词作主语补足语来理解

    motionless 只能作形容词用，其对应的副词形式是 motionlessly。

    motionless 的意思是“一动不动的，静止的”，其同义词是still。 

    在与 stand, sit, lay, remain 等动词连用时，应该使用形容词 motionless ，不能用副词形式 motionlessly。例如：The men stood motionless as Weir held his finger to his lips. 韦尔用手指按住嘴唇，那些人便一动不动地站住了。/ Graham remained motionless. 格雷厄姆一动不动。（摘自《朗文当代高级英语辞典》第5版）

“特被备注:

只要你能识别以上语言现象，并能熟练使用以上语言现象，你用以下哪种方式理解都可以.遇到语法观点有分歧，就选一种你最能接受那种观点去理解，去使用，语言最重要是能自己快速识别以上语言现象并熟练使用这一语言现象就OK 啦。

(1)双重谓语来理解以上语言现象

(2)形容词作方式/伴随状语来理解

(3)形容词作主语补足语来理解”