编译报错：File ended while scanning use of xxx

出现这个问题的原因是使用某些命令时，给出的参数不完整或者漏了半个大括号：

比如，

Runaway argument?

{adaptivity, dynamically changing environment, openness of these syst\ETC.

! File ended while scanning use of \UL@on.

回到原文去找，发现是  \uline{adaptivity, dynamically changing environment, openness of these systems 少了 ' } '。

又比如 \frac

$\frac{1}{x+1$

编译上边的命令，会提示 File ended while scanning use of \@frac，明显是第二个参数不完整。