当宾从的主语与主句的主语相同,且主句谓语动词是hope、wish、decide、agree、choose等时,从句可简化为不定式结构(eg:I hope that I can see you again.=I hope to see you again.);
当宾从的主语和主句的主语相同,主句的谓语动词是know、remember、forget、learn等,且宾从由what、who、which、when、where等词引导时,可把从句化简为“疑问词+不定式"(eg:She doesn't know what she should do.=She doesn't know what to do.);
当主句的谓语动词是ask、tell、show、teach等,且宾从的主语与主句的间接宾语一致时,宾从可简化为“疑问词+不定式“(eg:Can you tell me how I can get to the park?=Can you tell me how to get to the park?)
注意
只能用whether不能用if的情况:(1).在discuss或prep.之后;(2).whether or not;(3).whether to do.;