1. I want Tom to read a book. to do是否做宾补？即，Tom is to read a book.是成立的？ 

—— to read a book 是宾补。但是你不能用 is 去连接，去等同。

Tom is to read a book. 这是一个“将来时”的句子。“汤姆将要读书；应该读书”。

2. Fern was upstairs changing her sneakers. changing xxx是否是做主语补语？即，She is changing xxx成立？ 或者，上述理解是错的，有其他理解。

—— 理解错误。

Fern was upstairs changing her sneakers.

这是一个主系表结构（划线部分），changing her sneakers 是现在分词，作伴随状语。不是“主语补语”。

在不改变单词及数量的前提下，只在句中调换位置，就可以变出不同结构的句子，只是语义发生了变化。网友这个句子跟句3一致。仔细体会，好好揣摩下面6个句子：

1. Some children were playing on the sports ground.

翻译：一些孩子在运动场上玩耍。

分析：Some children (主语) were playing (谓语) on the sports ground (地点状语).

2. On the sports ground some children were playing.

翻译：在运动场上有些孩子在玩耍。

分析：On the sports ground (地点状语) some children (主语) were playing (谓语).

3. Some children were on the sports ground playing.

翻译：有些孩子在运动场上玩耍。

分析：Some children (主语) were (系动词) on the sports ground (表语) playing (伴随状语).

4. On the sports ground were some children playing.

翻译1：运动场上是一些在玩耍的孩子。

分析1：倒装句。On the sports ground (表语) were (系动词) some children (主语) playing (定语).

翻译2：有些孩子在运动场上玩耍。

分析2：倒装句。On the sports ground (表语) were (系动词) some children (主语) playing (伴随状语).

5. Playing on the sports ground were some children.

翻译：在运动场上玩耍的是一些孩子。

分析：倒装句。Playing (现在分词) on the sports ground (地点状语) were (助动词) some children (主语).

6. On the sports ground were playing some children.

翻译：一些孩子在运动场上玩耍。

分析：倒装句。On the sports ground (地点状语) were playing (谓语) some children (主语).