Only after her mother came to help her was she allowed to begin her project.

Only after her mother came to help her was she allowed to begin her project.

Only在这里是倒装句。

当句首状语由“only +副词”，“only +介词词组”，“only +状语从句”构成时，句子须倒装。例如：
Only yesterday did he find out that his watch was missing.
Only because there were some cancelled bookings did he get some tickets in the end.

谓语动词放在主语之前的这种语法现象称为倒装。从倒装的形式来看，可分为全部倒装(full inversion)和部分倒装(partial inversion)两种。前者是指整个谓语置于主语之前，而后者仅是指助动词、情态动词或be 动词等功能置于主语这前。

其实这两者没有很大区别，不过如果学会用倒装句，和外国朋友交流起来会更自然的。

用only就是到装 也不是正式些 要看看你强不强调某一个信息的 外国人比较喜欢到装 但具体到这句话 还是要看前后文的

Only这样的介词开头形成了倒装句 倒装句一般是用来强调某个方面的信息, 不过外国人,也像是中国人,有时也喜欢先说后面的再说前面的 有时却用来把句子拉长,就是3`4句合成一句