1." I don't know if he has returned home."
/ " I am not sure whether he has arrived at home."
whether/ if 在这里都是可以的.
2." How many books are left on the desk?" （书桌上还剩多少本书?）
很明显,这是一个被动语态,are+ left (leave的过去分词）.所以不是做形容词,而是一起做谓语动词.
还可以这样表达：" How many books are there being left on the desk?"
【公益慈善翻译团】真诚为你解答!How many books left are there on the desk ?left 过去分词 可以当形容词 作后置定语，这个 left 放后面很常见呀这个句子是不通顺的哦。所以left在这里不能这么用。left做后置定语的情况是，举个例子：“There are two books left on the desk."