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

be 是系动词的原形，在这里构成被动语态。 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。