Question

He will be watching her anxiously__she swims the long distan

Answer 1

during 是介词,它后面要跟宾语,表示一段时间,而宾语从句要有引导词（that, what, whether, when, why ...),这里she swims the long distance to England 显然不是一个宾语从句.

假如用during,这个句子要改成：

He will be watching her anxiously during the time when she swims the long distance to England.