英语语法：这句话中的“常用一个带连接代词/副词的不定式作宾语”是什么意思?

连接代词包括who,whom,whose,what,which,whoever,whomever,whichever,whatever,用于引导从句或不定式.
连接副词可分为两类,一类是用于连接句子或从句,常见的有therefore,besides,otherwise,however,moreover,still,thus,meanwhile等；另一类是用于引导从句或不定式,主要的有when,why,where,how 等.
连接代词/副词除了具有连接从句或不定式的作用外,在从句和不定式中也充当句子成分,如宾语、状语.其中宾语是名词性成分,用连接代词实现,而状语是副词性成分,用连接副词实现.
可用于引导不定式的连接代词：who,whom,whose,what,which
举例：
I asked him what to say next.= I asked him what I could say next.我问他接下来我该说些什么.(what在宾语从句和不定式中作宾语)
Show me what to do.= Show me what I should do.给我说说该做什么.(what在宾语从句和不定式中作宾语)
Did you ask her which to do first?= Did you ask her which you should do first?你问没问她该先做哪一个?(which在宾语从句和不定式中作宾语)
He doesn’t know whom to believe.= He doesn't know whom he should believe.他不知该相信谁.(whom在宾语从句和不定式中作宾语)
I wonder who to date.=I wonder who I should date.我拿不定该跟谁约会.(who在宾语从句和不定式中作宾语)
I've forgotten which path to follow.= I've forgotten which path I followed.我已经忘记我沿着哪条路来的.
这类用法通常可视为名词性从句的省略.这就是如你所说的"在动词tell,advise,show,teach,find out,decide,discuss,forget等后,常用一个带连接代词/副词的不定式作宾语."的情形.
可用于引导不定式的连接副词：when,why,where,how
举例：
When to stop swaddling their baby is a big question for all new parents.= For all new parents,when they can stop waddling their baby is a big question.对所有初为人父母者来说,什么时候开始停止襁褓包裹婴儿是一个大问题.(when在主语从句和不定式中做时间状语)
I couldn't see the reasons why to do so.= I couldn't see the reasons why you/we do so.我不知道这样做的理由是什么.(why在定语从句和不定式中做原因状语)
First,we have to decide where to live.= First,we have to decide where we will live.首先我们得确定在哪儿住下来.(where在宾语从句和不定式中做地点状语)
I'm not sure how to tell the difference between them.= I'm not sure how I can tell the difference between them.我不确定它们之间的区别.(how在宾语从句和不定式中做方式状语)
回到你的句子：We must decide whether to go or stay.= We must decide whether/if we should go or stay.我们得决定去还是留.(连接词whether在从句中不充当句子成分,只起连接作用).此时在不定式中不能用if连接.