but做介词用法but做介词,什么时候用to do,什么时候用do

ut 作介词 , “除了... ”
当主句中存在do 动词的任何形式, 介词but 后就省去to, 而只接do
如：
I couldn't do anything but sleep
I did nothing but wait
He had nothing to do but cry.
而I had no choice but to sleep （ had 属其他动词,接to do）
注： do 动词的任何形式包括：do 的现在时形式,过去式形式,完成时形式（have done, had done.) 将来时will do ,would do 等.
巧记：but 前do后无to

如果前部分有动词 do， 作“做” 讲， bur 后接 do
如果前面没有do"做“ 用 to do
I can do nothing but wait.
There is nothing for me to do but watch TV
看下句 就没 do
I have no choice but to wait
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