为了让你更容易理解我的解答，请按照以下思路考虑：

1. He seated himself on the chair.

2. He was seated on the chair.

3. People saw him seated on the chair.

4. He was seen seated on the chair.

【讲解】你的句子就是句4的格式。

句1：seat是及物动词，意为“使某人坐下”。

句2：是句1的变式（被动式），请看后面的详解。

句3：see sb. + 过去分词。

句4：是句3的变式（被动式）。

【反身动词详解】

英语语法有一条规则：就是“现在分词表主动；过去分词表被动”，这已经是大家公认的规则。

然而，任何事物都有其特殊的现象和内在规律，英语亦然。在有些情况下，英语的被动语态并不表示被动，而是主动。例如：

John was seated in the chair, writing a letter. 约翰坐在椅子上，正在写信。

The lady who was dressed in red clothes was his aunt. 穿红色衣服的这个女士是他的姨妈。

有网友可能会问：汉语中的“穿衣”和“坐下”，本来是主动的意思，在英语中为何用被动形式be dressed in或be seated on？

“被动语态表主动”，这是一个很有意思的问题，该现象的本质皆缘于反身动词跟反身代词的用法。主语和宾语同为一人，主语是动作的发出者，又是动作的接受者，这就出现了被动语态或过去分词表示“主动”含义的语言现象。以上列举的含有反身动词（代词）的例句，有些去掉oneself 变成被动句，意思保持不变，仍表示主动含义。除了seat和dress外，又如：

He quickly accustomed himself to this new way of life.

=He was quickly accustomed to this new way of life.

他很快就习惯了这种新的生活方式。

You will have to accommodate yourself to the changed situation.

=You will have to be accommodated to the changed situation.

你必须适应变化了的形势。

She adapted herself to the new climate quickly.

=She was adapted to the new climate quickly.

她很快就适应了这种新气候。

He can't adjust himself to the whirl of modern life in this big city.

=He can't be adjusted to the whirl of modern life in this big city.

他无法适应这个大都市的现代生活的忙碌。

The old man addicted himself to smoking.

=The old man was addicted to smoking.

那位老人吸烟成瘾。

Nowadays some young people abandon themselves to drinking.

=Nowadays some young people are abandoned to drinking.

现如今，一些年轻人沉湎于饮酒。

You will have to bury yourself in your studies if you want to pass the exam.

=You will have to be buried in your studies if you want to pass the exam.

如果你想通过考试，就得专心致志地学习。

Robert charged himself with the task of overcoming a technical difficulty.

= Robert was charged with the task of overcoming a technical difficulty.

罗伯特承担了攻克技术难关的任务。

He hid himself in the woods.

= He was hidden in the woods.

他藏在森林中。

She should concern herself with your health.

= She should be concerned with your health.

她应关心你的身体。

After graduation he continued to devote himself to the research.

=After graduation he continued to be devoted to the research.

毕业之后他继续致力于研究工作。

He dedicated himself to the cause of science.

= He was dedicated to the cause of science.

他献身科学事业。

She distinguished herself by her coolness and bravery.

=She was distinguished by her coolness and bravery.

她因头脑冷静、敢作敢为而为人称道。

She dressed herself in a red dress at the party testerday.

=She was dressed in a red dress at the party testerday.

昨天在晚会上，她穿上了一件红色的连衣裙。

For years he has drowned himself in the study of English literature.

=For years he has been drowned in the study of English literature.

好多年来他一直钻研英国文学。

He wants to engage himself in foreign trade.

=He wants to be engaged in foreign trade.

他想从事对外贸易。

When I lose myself in the study, I can't hear anything.

=When I am lost in the study, I can't hear anything.

当我沉浸于学习中，我什么也听不见了。

He occupied himself with various research projects.

=He was occupied with various research projects.

他终日从事各种研究计划。

I opposed myself to the training plan.

=I am opposed to the training plan.

我反对那项训练计划。

I am preparing myself for the long ride on the bus to work.

=I am prepared for the long ride on the bus to work.

我正准备踏上耗时的上班之旅。

I restrict myself to smoking two cigarettes a day.

=I am restricted to smoking two cigarettes a day.

我每天只抽两支烟。

But after he gets over the shock, he usually resigns himself to his fate and then becomes as happy as normal boys.

=But after he gets over the shock, he is usually resigned to his fate and then becomes as happy as normal boys.

可是这种难受不安过去后，他也只有听天由命，现在比一般正常的儿童还快乐。

He was finally able to rid himself of all financial worries.

He was finally able to be rid of all financial worries.

他终于能摆脱了所有的财政忧虑。

They seated themselves on a bench that stood by the lake.

They were seated on a bench that stood by the lake.

他们在湖畔的一条长凳上坐下。

He threw himself into work once he got back from his holidays.

=He was thrown into work once he got back from his holidays.

他度假一回来就投身于工作。

I don't want to associate myself with them any more. 

=I don't want to be associated with them any more. 

我不愿再和他们交往了。

He isolated himself from the masses.

= He was isolated from the masses.

他脱离了群众。