I defined a function $\mathrm{fun}[a \underline{\hspace{0.5em}} \,]:=a^2$, but it showed that "No Wolfram Language translation found". Now I found that I can name a function single letter like $f$, $g$ or a letter followed by a number like $f1$, $g1$. But I saw that some other people could use the function names with more than one letter. I have no idea why this issue exist. I tried to restart Mathematica but it did not work. I am doubting whether that is because of the edition of the Mathematica I use, I got the access through our university.