“I don't what to do.”中的what to do充当什么成分,是宾语从句吗?

是I don't know what to do
作宾语,不是宾语从句,what to do就是个带疑问词的不定式短语,
不是句子,主语谓语都没有.
不定式作宾语有两种情况：一是及物动词后直接跟带to的不定式,二是“动词+疑问词 +带to的不定式”.
⑴“动词+带to的不定式”结构：常见的以不定式作宾语的动词有afford,agree,believe,decide,fail,hope,want,plan,choose,prepare,forget,remember,begin/start,learn,promise,refuse,wish等.如：
I can’t afford to buy a house.
⑵“动词+疑问代（副）词+不定式”结构：常见的这类动词有advise,decide,forget,know,learn,remember,see,tell,think,teach,wonder,understand,guess,explain等.这类疑问代（副）词what,when,which,where,how,whether等（不包含why）.如：
She does not know how to go there.
He will tell you when to start.