as为连接词,作“以...方式”解时,后面连接的句子不完整,省略了,可是哪些部分可以省,哪些不能省呢

第三句话 为什么要有to
是因为 to 是动词不定式的标志 标志要有 但是具体的动词就省略了 避免重复
Why didn't you catch the last bus as I told you to（catch）?
你怎麽不听我的话赶乘末班公共汽车呢?
关于动词不定式的省略
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动词不定式的省略
　　为了避免重复,我们常常把作宾语、宾补和谓语动词的一部分的不定式省略,只保留动词不定式的符号to.现将常见省略不定式的几种情况通过实例加以简析,供大家参考：
　　(1)—How about coming to my house?
　　—I’d love to if it doesn’t give you so much trouble.
　　在hope,like,love,promise,want,wish等词后作宾语的不定式常省略.再如：
　　You may go if you want to.
　　She can get a job if she hopes to.
　　—How about going hunting with me tomorrow?
　　—I’d like to,but I have no time.
　　(2)Don’t close the window until I ask you to.
　　在allow,ask,tell 等词后作宾语补足语的不定式常省略.再如：
　　Don’t touch the light unless your mother allows you to.
　　Don’t plant potatoes until the peasant tells you to.
　　(3)He didn’t want to hand in his composition,but he had to.
　　在be able to,be going to,have to,need to,enough to,used to等后的不定式需省略.再如：
　　I don’t sing much,now,but I used to a lot.
　　If you don’t want to say anything at the meeting,you don’t need to.
　　She didn’t go out last night,because she was afraid to.
　　在afraid,glad,happy,pleased,sorry等词后作状语的不定式常省略.再如：—Will you go with me to see the film tonight?
　　—I’ll be glad to.

因为第三句to后面省略的是 动词不定式短语“catch the last bus”
一般动词不定式的形式才可以这样省略的。