Though he was very young, he knew how to behave well.

这种句子不属于倒装，而是表语前置。

① Young though he was, he knew how to behave well.

② Very young though he was, he knew how to behave well.

表语前置本身就是强调，因此没有必要再使用very。故第一句正确，第二句欠妥。

③ Though he was very young, he knew what the right thing was.

④ Though he was very young, he knew what was the right thing.

这二句均正确，因为这是确认性表语，主语和表语可以互换句法功能。例如：

This is the right thing. 改为问句：What is the right thing？ what就主语提问，句子无倒装。去掉问号即为名词性从句what is the right thing

The right thing is this. 改为问句：What is the right thing? what就表语提问，is 倒装。去问号、去倒装，则得到名词性从句what the right thing is

⑤ Though he was very young, he knew what the right thing to do was.

⑥ Though he was very young, he knew what was the right thing to do.

以上二句均正确。道理同第三、四句，确认性表语。

⑦ Though he was very young, he knew what the right thing was to do.

错误，应改为第5或第6句。

⑧ Though he was very young, he knew what was right.

正确。形容词作归属性表语。