什么词后只加todo什么后只加doing答:help learn long 渴望 mean manage offer plan pretend refuse tend undertake expect hate intend 使役动词 let, have, make后,感官动词 see, watch, look at, notice , observe, hear, listen to, smell, feel, find 等后.注意:被动语态中不能省去to.例如:I saw him dance. 我看见他跳舞....