refer to后跟doing，因为to是介词，后加名词/代词/ving.

Later it came to refer to studying under very hard conditions.  

之后指在困难的条件下周旋耐劳研习。

有些带 to的动词，后加doing，因为to是介词，这些动词短语有：

1) admit to doing sth.承认做了某事

(2) apply to doing sth.适用于做某事

(3) object to doing sth.反对做某事

(4) see to doing sth.负责做某事

(5) stick to doing sth.坚持做某事

(6) take to doing sth.喜欢上做某事,逐渐习惯做某事

（7）get down to doing sth 开始做…