1." I don't know if he has returned home."
/ " I am not sure whether he has arrived at home."
whether/ if 在這里都是可以的.
2." How many books are left on the desk?" （書桌上還剩多少本書?）
很明顯,這是一個被動語態(tài),are+ left (leave的過去分詞）.所以不是做形容詞,而是一起做謂語動詞.
還可以這樣表達：" How many books are there being left on the desk?"
【公益慈善翻譯團】真誠為你解答!How many books left are there on the desk ?left 過去分詞 可以當形容詞 作后置定語，這個 left 放后面很常見呀這個句子是不通順的哦。所以left在這里不能這么用。left做后置定語的情況是，舉個例子：“There are two books left on the desk."