You could tell that they were itching to leave.

were 是 be 动词的过去式复数形式，表示"是"或"存在"的状态。 itching 是动词 itch 的现在分词形式，itch 本意是"发痒"，引申为"渴望、迫切想要"。 to 在这里是不定式符号，与动词原形连用表示目的或意图。 were itching to 这个短语表示"非常渴望做某事"，形容一种迫不及待想要离开的状态。