\documentclass{article}
\usepackage{xparse}
% example from documentation at: https://mirror.las.iastate.edu/tex-archive/macros/latex/contrib/l3packages/xparse.pdf
\NewDocumentCommand {\conjugate} { m O{#1ed} O{#2} } {(#1,#2,#3)}
\begin{document}
\conjugate {walk}            % => (walk,walked,walked)

\conjugate {find} [found]    % => (find,found,found)

\conjugate {do} [did] [done] % => (do,did,done)
\end{document}