I am very doubtful about whether you were there when it was held.

doubtful 是形容词，由名词 doubt（怀疑）加上后缀 -ful（充满...的）构成，意思是"充满怀疑的、不确定的"。 在句中表示说话者对"你是否在场"这件事持怀疑态度。