His qualifications were in doubt until he produced the necessary certificates.

were 是动词 be 的过去式，在句中作系动词，连接主语和表语 in doubt。 in 是介词，表示"处于某种状态"。 doubt 是名词，意思是"怀疑、疑问"。 be in doubt 是一个固定短语，表示"受到质疑/存在疑问"，描述主语处于被怀疑的状态。