“She dropped the tray when I spoke to her.”和“She dropped the tray when I was speaking to her.”两句都对,后一句强调“我正在说话的时候”.不过,在(过去)进行时态的结构中,英语更习惯说“I was speaking to her when she dropped the tray.”
而且,speak to sb（about sth）还有个意思就是“（就某事）劝告,责备”.也可能翻译为：我责备她的时候她摔下盘子.