my \begin won't work

Question

when I try to use a \begin{left} and \begin{right}, they get mixed up. this is my code

\begin{left}
    \begin{tabular}{l|c|c}
        Monophthongs & Front & Back \\
        \hline
        Close & i iː & u uː \\
        Mid & e eː & o oː \\
        Open & a aː
    \end{tabular}
\end{left}
\begin{right}
    \begin{tabular}{c|c}
         Diphthongs & a & e & i & o & u \\
         \hline
         a &  & ae &  &  & au \\
         e &  &  & ei &  & eu \\
         u &  &  & ui &  &  \\
         i &  &  &  & io & iu \\
         o &  & oe &  &  &  \\
    \end{tabular}
\end{right}

Answer 1

I guess, you wanted to get both tables to show up side by side. To achieve this, remove the left and right environment and make sure, there is no empty line between the first \end{tabular} and the following \begin{tablar}. To get a small horizontal white space between both tables, I used \quad here, but you could also usa any other command for horizontal spacing of your choice here.

To make sure, both tables are top aligned with respect to each other, I added [t] to both tabular environments. Other options include [c] for vertically centered and [b] for bottom alignment.

Lastly, I also added the missing column specifiers in your second table. Using c|c you only declared 2 columns, but used a total of 6 columns in your table. I thus changes that to c|ccccc. Feel free to change the column specifiers to suit your needs.

If you want to add a caption for both tables of an individual caption for each table, take a look at https://tex.stackexchange.com/a/597566/134144 for an overview over different options and approaches on how to do that.

\documentclass{article}
\begin{document}
\begin{tabular}[t]{l|c|c}
    Monophthongs & Front & Back \\
    \hline
    Close & i iː & u uː \\
    Mid   & e eː & o oː \\
    Open  & a aː
\end{tabular}

\quad
    \begin{tabular}[t]{c|ccccc}
         Diphthongs & a & e & i & o & u \
         \hline
         a &  & ae &    &    & au \
         e &  &    & ei &    & eu \
         u &  &    & ui &    &    \
         i &  &    &    & io & iu \
         o &  & oe &    &    &    \
    \end{tabular}
\end{document}