'Themself' is the reflexive word you use when you are talking about ONE person whose sex you do not know or do not need to say. You already use singular 'they' for that person — 'someone left their bag' — and 'themself' is what happens when that person does something to themself: 'anyone who hurts themself should tell a teacher.' Why does it exist? Because English never gave us a good he-or-she reflexive. The old advice was 'himself' for everyone, which leaves out half the world; then 'himself or herself', which is heavy; then 'themselves', which sounds plural when you mean one person. 'Themself' fills that hole neatly: singular 'they' plus singular 'self'. Here is the honest part. Many people still think it is a mistake, and some teachers and exam markers will mark it wrong. That judgement is loosening — dictionaries now list it, style guides increasingly allow it, and it is also used by and for people who use 'they' as their own pronoun. But the safest thing in a school exam or a formal letter is still 'themselves' with singular they ('someone hurt themselves'), or rewriting to the plural ('students who hurt themselves'), which dodges the whole argument. So: understand it, expect to see it in real English, and know that using it is a small choice that some readers will notice.

（單數的）他/她自己——不指明性別時用的反身代名詞。中文本來就不必說明性別，所以這個困擾對台灣學生是英文獨有的；正式場合寫 themselves 較保險