用which或what.但也不是固定不变的。
例句：Donald Trump is the 45th president of the USA.提问: Which (term) of president is Donald Trump?一般用"Which"来提问序数词，但英语和中文无法一一对应，比如:What grade are you in? 你读几年级?I am 6th grade.我六年级。在这个句子中，不用"Which" 用"what吾能弭謗矣……」英语没有「第几」的通用问法，只能含糊地问「什么数」，「哪个」，或者迂曲地表达：What number is President Donald Trump?What number President is Donald Trump?Sequentially， which president is Donald Trump? ——按次序讲，……Where does Donald Trump fall in the sequence of US presidents? ——在美国总统的序列中，Trump 排在哪儿？What is Trump's number in the order of US Presidents?
