5 Next came a horse, swimming bravely, ...

Next came a horse 是倒装语序, 正常语序: a horse came next.

6 We realized that they must have become unduly frightened by the rising flood, for their house, which had sound foundations, would have stood stoutly even if it had been almost submerged.新概念英语复读机

用must 表示猜测, 猜测过去的事用 must have done;

the rising 修饰 flood; 表示"不断上涨的洪水"

for 引导原因状语从句

sound adj.  健全的,可靠的

submerged  V 淹没

7 One had winter we watched the river creep up the lower meadows.

creep up  作宾语补足语,他其实是一个不带 to 不定式, 因为动词是watch ,所以省略to

重点结构表达：

be related to...  与...相关 

hard by  紧挨着

spell...  for somebody   给...带来...

make a living  谋生

make up   补足,拼凑

take turns  轮流

floating down  顺流而下

waded down  趟过

in the hope of   抱着...希望

pull ... out of   把...拖出...

creep up  慢慢地爬上