____it is to swim in the river in summer!

一、首先是明白funny 与 fun 词性不同：funny 是形容词,意为“有趣的,滑稽的,奇怪的”；fun 是名词,意为“有趣的事”.
二、两者含义上也有差别：funny 表示“有趣的,滑稽的”,它指的是一种滑稽可笑的“有趣”,侧重点是让人觉得好玩或发笑.而 fun 则主要指“有趣,好玩”,不像funny 那样强调“滑稽”.
三、比较并体会以下例句：
It is more fun to go with someone than to go alone. 偕伴同去比自己独自去好玩.
It's funny to see a man walk with his hands and head. 看见一个人倒立走路很滑稽.

it在此句中可换成to swim in the river in summer
也就是说原句 = ________ to swim in the river in summer is!
现在你在选的话会选哪个？

How funny不对，应是How fun

funny 意思是好笑
你意思没整好.