关于Bed is used to sleep on.我知道这句话的on不少,因为它构成了一个动宾关系,即Sleep on the bed.所以on不能省略.但请看这句话：Sweater is used to keep warm.这句话怎么不说成是：Sweater is used to keep warm wit

关于Bed is used to sleep on.我知道这句话的on不少,因为它构成了一个动宾关系,即Sleep on the bed.所以on不能省略.但请看这句话：Sweater is used to keep warm.这句话怎么不说成是：Sweater is used to keep warm wit
关于Bed is used to sleep on.
我知道这句话的on不少,因为它构成了一个动宾关系,即Sleep on the bed.所以on不能省略.但请看这句话：Sweater is used to keep warm.这句话怎么不说成是：Sweater is used to keep warm with呢?要知道我们是用毛衣来保暖的呀with the sweater!

关于Bed is used to sleep on.我知道这句话的on不少,因为它构成了一个动宾关系,即Sleep on the bed.所以on不能省略.但请看这句话：Sweater is used to keep warm.这句话怎么不说成是：Sweater is used to keep warm wit
是否需要介词,主要是看名词和不定式短语之间的逻辑关系,如果名词是逻辑宾语,必要的介词不可或缺,如果是是逻辑主语,名词是支配不定式动作或状态的行为者,与是否有介词毫无干系.
1.Bed is used to sleep on
bed 是 sleep on 的逻辑宾语,而 sleep 是不及物动词,如果缺少介词 on,逻辑宾语 bed 就会失去依从,也就失去宾语的地位.
再如：
a pen is used to write with 即“用笔写字”
a chair is used to sit on ,即“坐在沙发上”
food is used to live on (依靠食物为生)
2.Sweater is used to keep warm
keep warm 是系表结构,形容词 warm 用来说明 sweater 的特征,；将其改回主动即为 to use sweater to keep warm,由此看出,sweater 是 keep warm 的逻辑主语,没有使涉及到介词问题.
再如一下也属主谓关系：
some chair are used to place in the roomn
food is used to supply a need 生

这里最大的问题不在于介词如何，在于前面用的词是否是及物动词，如果是不及物动词，一定要有介词。比如说前面的SLEEP.可是KEEP本身就是及物动词。