I always hear Chinese people making sentences using 而已 - but I've never quite, 100%, understood how to use this in sentences myself. What is the correct way to apply it, and what is the English equivalent?
“而已” is always used at the end of a sentence and with words like “仅仅”,"只","不过". A similar word in Chinese is "罢了"(actually, this word comes from 满语). You use the structure "......(part A),不过......(part B)而已" to emphasize the expression that A is just limited within B. You could only use "不过", also, and "而已“ weakens your mood and sometimes expresses that you don't have help to (or don't take care of) B.
Your girlfriend and you are watching a magic performance. The magician is going to stab a sword into the assistant's body and your girlfriend gets scared. You would say:"别紧张，这不过是魔术表演而已。"(Don't worry, this is just a magic performance[so you don't have to get worried about it])
The famous writer in modern times, 鲁迅(lu3 xun4) has a novel collection called 《而已集》.In the preface, he wrote:"连‘杂感’也被‘放进了应该去的地方’时，我于是只有‘而已’而已" (while my "random thoughts“ are put into the place where it is supposed to go(meaning, he is not allowed to publicize his thoughts, criticisms), I could only say "而已")to show his helpless and sadness.
The equivalent for the structure "不过......而已" would be "only","just".
The easy way to understand this is to use an oral English sentence rather than something formal. This is commonly used to mean "...and that's all".
For example in English you can say "It's just an exam and that's all".
In Chinese this could be said: "这不过是一个考试而已" Zhè bùguò shì yīgè kǎoshì éryǐ