【句子】
That xenon could not form chemical compounds was once believed by scientists.
科学家曾相信：氙气是不能形成化合物的。
【解析】
1.form
1)n. 形式，形状，表格，良好的健康状态，表现状态，常规
①The team were on excellent form throughout the whole competition.
这个队在整个比赛过程中一直处於极好的竞技状态。
②I've got no record of this horse's form.
我没有这匹马的情况记录。
2)v. 形成，建立
His research formed the basis of his new book.
他的研究成果是他这本新书的基础。
2.once
