I do not know what to reply .请问为什么what 后面要加一个to ,是词组,还是什么语法,

I do not know what to do.同义句子I don't know_______ ______ _____ _____. I do not know what you I do not know what you i don't know what to do Don't know what i'd do What ,I do not know what you mean什么意思 what's in the newspaper today?I do't know.I____what's in the newspaper today?I do't know.I____（not/read it/yet） I do not know what I will do.写出同义句I do not know what _________ ________. 同义句1I do not know what I can do now.I do not know what ( ) ( )now. i don not know what i should do next 同义句:I don't know what I can do .转换：I don't know what ( ) ( ). I do not know I do not know 同义句转换：1.I do not know what Ishuld do next.I do not know ( ) ( )do next. I don't know what I should do 同义句 I don't know what i should do（同义句) what should I do 同义句I don't know ( )( )( ). I understand,but,I don't know what to do?