"他们很失落"可以用英文表达为 "they are very disappointed"。

"失落"是一个英文词汇，它的意思是指感到沮丧、失望或悲伤。这个词的来源是源自拉丁语"disappoint"，它的意思是"失望"或"挫败"。

下面是一些用 "they are very disappointed" 这句话的例句，以及它们的中文翻译：

"They are very disappointed that they didn't get the job." (他们很失落没有得到那份工作。)
"They were very disappointed when they found out that the concert was sold out." (当他们发现那场音乐会售罄时，他们很失落。)
"They were very disappointed when they heard the news." (他们听到这个消息时很失落。)