Little Bob is too shy to speak if he is not spoken to

填上：unless spoken .
原句的意思是：小Bob太怕羞了,如果没有人跟他说话,他都不会主动说的.
unless是“除非”.本来完整的同义句应该是
Little Bob is too shy to speak unless he is spoken to.小Bob太害羞了,除非有人跟他说话,否则他是不会主动开口的.
unless引导的从句中：
1、主语和主句的主语一样,都是指Bob
2、从句谓语是be动词
满足以上两个条件,可以省略主语和be动词,变成Little Bob is too shy to speak unless spoken to.