不定式 省略toRober will collect us by 10 o’clock,he promise (to)i

不定式 省略to
Rober will collect us by 10 o'clock,he promise (to)
i admit that i took her watch,but i didn't mean to.
为什么第一句to可以省略,第二句to不能省略?这两句有什么区别吗?
第一句完整的是,he promise to collect us by 10 o'clock
第二句完整的是,i didn't mean to took her watch .
感觉第二句 to 跟mean 紧密一点
第一句 to collect us 做宾语,第二句to took her watch 是不是也做宾语还是做状语?
【补充1】
对 anlaowu555 - 跟mean相似的还有expect 而expect 和promise一样也可以有expect that 怎样从语法上来解释两个句子的区别。
【补充2】
这是advanced grammar in use 上的例子，处mean ,expect ,还有afford to ,be able to,choose to,deserve to ,expect to ,fail to ,hate to ,hope to ,love to ,need to ,prefer to.这些to都不能省略，vs_jndk 说的mean to 是固定短语是不是意思是mean to 是phrasal verb 但是查字典知mean to 不是 phrasal verb,并且 其他的这些动词+to 也不是phrasal verb。
【补充3】
对charlie254 Rober will collect us by 10 o'clock。这句中的will 不是关键，另外一个句子没有will
you were supposed to buy some sugar.sorry i forgot (to).
“i forgot 宾语是you were supposed to buy some sugar整句句子” 为什么？sorry i forgot to ,可以带to

第一句：语法规定情态动词（这里是will）和助动词后面不定式省略to
第二句：为了避免重复前面的动词,后面通常省略不定式,保留to
that i took her watch是宾语从句,作宾语
i forgot 宾语是you were supposed to buy some sugar整句句子,哪来的不定式,语法关于不定式省略to就这么几条,还不懂吗
补充的没理解,你要问什么区别